Control circuit and switching power supply

ABSTRACT

A control circuit according to an embodiment of the present invention is configured to control a switching element of a switching power supply. The control circuit includes a comparator having a first input terminal configured to receive an output voltage of the switching power supply. The comparator has a second input terminal that is connectable to a positive terminal of a reference voltage source. The comparator has an output. The output brings the reference voltage to a first voltage while the output signal takes a first voltage level. The output brings the reference voltage to a second voltage while the output signal takes a second voltage level. The constant voltage source has a positive terminal connected to a negative terminal of the reference voltage source and a ground of the comparator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention are related to a control circuitwhich controls an output voltage, and a switching power supply.

The present application claims priorities on Japanese Patent ApplicationNo. 2015-027366, filed Feb. 16, 2015, Japanese Patent Application No.2015-029525, filed Feb. 18, 2015, Japanese Patent Application No.2015-029954, filed Feb. 18, 2015, and Japanese Patent Application No.2015-169595, filed Aug. 28, 2015, the contents of which are incorporatedherein by reference.

2. Description of Related Art

As a method of controlling a switching power supply, hysteresis controlusing a hysteresis comparator is known (see JPH03-293965A andJP2014-57476A). In the hysteresis control, a reference voltage which isinput to a non-inverting input of a comparator takes a first highvoltage when the output of the comparator takes a high level (H level)and a second low voltage when the output of the comparator takes a lowlevel (L level). The difference between the first voltage and the secondvoltage is a hysteresis width. When a voltage in which an output voltageis resistance-divided is brought to be lower than the reference voltageof the second voltage, the output of the comparator is brought to be atthe high level (H level) to start a drive period, causing the referencevoltage to be brought to the first voltage. When a current which islarger than a load current is supplied from the switching power supplyto an output capacitor to raise the output voltage during the driveperiod to bring the voltage in which the output voltage isresistance-divided to be higher than the reference voltage of the firstvoltage, the output of the comparator is brought to the low level (Llevel) to complete the drive period, transitioning to a stop period. Atthis time, the reference voltage is brought to be the second voltage. Inthe stop period, no current is supplied from the switching power supplyand a load current is supplied from the output capacitor, so that theoutput voltage decreases. JP2014-05476A discloses that aphase-compensating capacitor C2 which is connected in parallel with aresistance which voltage-divides an output voltage is further added tomake more stable control possible.

Moreover, burst control in which switching is performed a plurality oftimes during a period in which a hysteresis comparator takes a highlevel (H level) is known as a control method in which the hysteresiscomparator is used. JP2007-181389A discloses that a hysteresiscomparator which compares an output voltage and a reference voltage anda circuit which turns off a gate voltage during a period when a currentof a switch element reaches a value are provided. As a process isrepeated in which, during the period in which the hysteresis comparatoris at the high level (H level), the gate voltage is turned on to causethe current to reach the value and turn off the gate voltage, the gatevoltage is again turned on as the period in which the hysteresiscomparator is at the high level (H level) continues, and the currentagain reaches the value to turn off the gate voltage, switching isperformed a plurality of times during the period in which the hysteresiscomparator is at the high level (H level). When the hysteresiscomparator is at the low level (L level), the gate voltage does not turnon, so that switching stops. During the period in which the hysteresiscomparator is at the high level (H level), switching is repeated, sothat supplying a current which is larger than a load current from theswitching power supply to the output capacitor raises the outputvoltage. During the period in which the hysteresis comparator is at thelow level (L level), switching is stopped, so that no current issupplied from the switching power supply and the load current issupplied from the output capacitor, decreasing the output voltage.

Problems of the above-described control methods include that, when theoutput voltage is quite high compared to the reference voltage and avoltage-dividing ratio when the output voltage is divided by resistancedividing to around the reference voltage is large, an output voltageripple is brought to the product of the voltage-dividing ratio and thehysteresis width, which is the difference between the first voltage andthe second voltage. To prevent erroneous operation due to noise, thehysteresis width may not be decreased unlimitedly. Moreover, in thephase-compensating capacitor C2, which is disclosed in JP2014-057576A,it is difficult to control output voltage fluctuations to the hysteresiswidth. It is desirable for the output voltage ripple to fall within thehysteresis width.

Moreover, a soft start function for slowly raising the output voltagefor preventing an inrush current at the time of activating the switchingpower supply is known.

For example, WO2005/101629A discloses that a capacitor voltage for softstart is instantaneously charged to the same level as a triangular wavefor PWM from a beginning of activation to shorten the period from thebeginning of the activation to a beginning of a rise of an outputvoltage.

Moreover, JP2013-240159A discloses that a soft start voltage isincreased rapidly from a beginning of activation to detecting a PWMpulse signal to shorten a period from the beginning of the activation toa beginning of a rise of the output voltage.

Problems of the above-described control methods include that, when acapacitor with a large static capacitance is used for the controlcircuit of the switching power supply, it takes time for a voltage ofthe capacitor to be charged from 0V at the time of activation to asteady state value, so that it takes time for the output voltage toreach a target value at the time of activation.

SUMMARY OF THE INVENTION

In view of problems of the related-art, one object of aspects of thepresent invention is to provide a control circuit and a switching powersupply that make it possible to control an output voltage ripple of theswitching power supply to within a desired range.

To achieve the above-described object, a control circuit according toone aspect of the present invention is configured to control a switchingelement of a switching power supply, the control circuit including: acomparator having a first input terminal configured to receive an outputvoltage of the switching power supply, the comparator having a secondinput terminal that is connectable to a positive terminal of a referencevoltage source, the comparator having an output, the output brings thereference voltage to a first voltage while the output signal takes afirst voltage level, the output brings the reference voltage to a secondvoltage while the output signal takes a second voltage level; and aconstant voltage source having a positive terminal connected to anegative terminal of the reference voltage source and a ground of thecomparator.

To achieve the above-described object, a control circuit according toanother aspect of the present invention is configured to control aswitching element of a switching power supply having an output terminalconnected to a positive terminal of a constant voltage source, thecontrol circuit including: a comparator having a first input terminalthat is connectable to a negative terminal of the constant voltagesource and the comparator having a second input terminal that isconnectable to a reference voltage source, the comparator having anoutput, the output brings the reference voltage to a first voltage whilethe output signal takes a first voltage level, the output brings thereference voltage to a second voltage while the output signal takes asecond voltage level; and a controller configured to control theswitching element based on the output of the comparator.

In view of problems of the related-art, one object of aspects of thepresent invention is to provide a control circuit and a switching powersupply that make it possible to control output voltage fluctuations suchas an output voltage ripple, a static load fluctuation, a dynamic loadfluctuation, a static input fluctuation, a dynamic input fluctuation,etc., to fall within a desired range.

To achieve the above-described object, a control circuit according toanother aspect of the present invention is configured to control aswitching transistor of a switching power supply, the control circuitincluding: a third resistor and a fourth resistor that are connected toeach other to voltage-divide an output voltage of the switching powersupply; a comparator having a first input terminal configured to receivethe output voltage voltage-divided by the third resistor and the fourthresistor, the comparator having a second input terminal configured toreceive a reference voltage; and a controller configured to control theswitching transistor based on an output signal of the comparator,wherein the control circuit is configured to bring the reference voltageto a first voltage while the output signal of the comparator takes afirst voltage level, and the control circuit is configured to bring thereference voltage to a second voltage while the output signal of thecomparator takes a second voltage level, and the third resistor isconnectable between a positive terminal of an output terminal of theswitching power supply and the first input terminal of the comparator;the control circuit further including: a capacitive element that isconnectable to the third resistor in parallel with each other, wherein,where a resistance value of the third resistor is R1, a resistance valueof the fourth resistor is R2, a minimum switching frequency of theswitching transistor is Fmin, and a static capacitance of the capacitiveelement is C1, Equation (1)

$\begin{matrix}{{C\; 1} \geqq \frac{{R\; 1} + {R\; 2}}{2\pi \; F_{m\; i\; n}R\; 2^{2}}} & (1)\end{matrix}$

is met

To achieve the above-described object, a control circuit according to afurther embodiment is configured to control a switching transistor of aswitching power supply, the control circuit including: a third resistorand a fourth resistor that are connected to voltage-divide an outputvoltage of the switching power supply; a comparator having a first inputterminal configured to receive the output voltage voltage-divided by thethird resistor and the fourth resistor, the comparator having a secondinput terminal configured to receive a reference voltage; and acontroller configured to control the switching transistor based on anoutput signal of the comparator, wherein the control circuit isconfigured to bring the reference voltage to a first voltage while theoutput signal of the comparator takes a first voltage level and thecontrol circuit is configured to bring the reference voltage to a secondvoltage while the output signal of the comparator takes a second voltagelevel, and the third resistor is connectable between a positive terminalof an output terminal of the switching power supply and the first inputterminal of the comparator; the control circuit further including: acapacitive element and a switch element that are connected in parallelwith the third resistor and are mutually connected in series; and an OFFperiod detection circuit configured to detect that the output of thecomparator continually takes the second voltage level in a period whichis longer than a predetermined period, wherein the OFF period detectioncircuit is configured to open the switch element and bring the referencevoltage to a third voltage when the OFF period detection circuit detectsthat the output of the comparator continually takes the second voltagelevel in the period which is longer than the predetermined period.

In view of problems of the related-art, one object of aspects of thepresent invention is to provide a control circuit and a switching powersupply that make it possible to cause an output voltage to reach atarget value in a short time at the time of activation.

To achieve the above-described object, a control circuit according toone aspect of the present invention is a control circuit of a switchingpower supply, the control circuit including: a capacitive element; aswitch element that is connectable to the capacitive element; and anoutput voltage detection circuit configured to compare an output voltageof the switching power supply with a predetermined value, close theswitch element to charge the capacitive element when the output voltageis lower than the value, and open the switch element when the outputvoltage is higher than the predetermined value.

To achieve the above-described object, a control circuit according toone aspect of the present invention is a control circuit of a switchingpower supply, the control circuit including: a capacitive element; aswitch element that is connectable to the capacitive element; and anoutput voltage detection circuit configured to close the switch elementto charge the capacitive element when the switching power supply isactivated, and compare an output voltage of the switching power supplywith a value to open the switch element when the output voltage ishigher than the predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining a circuit configuration related to aswitching power supply which controls an output voltage by hysteresiscontrol according to a first embodiment of the present invention;

FIG. 2 is a timing waveform diagram for explaining an operation relatedto control of the switching power supply in FIG. 1;

FIG. 3 is a circuit diagram illustrating a specific example of a controlcircuit related to FIG. 1;

FIG. 4 is a diagram for explaining the circuit configuration related tothe switching power supply which controls the output voltage by thehysteresis control according to a second embodiment of the presentinvention;

FIG. 5 is a timing waveform diagram for explaining the operation relatedto the control of the switching power supply in FIG. 4;

FIG. 6 is a circuit diagram illustrating one specific example of thecontrol circuit related to FIG. 4;

FIG. 7 is a circuit diagram illustrating another specific example of thecontrol circuit related to FIG. 4;

FIG. 8 is a circuit diagram illustrating a configuration of a switchingpower supply according to a third embodiment of the present invention;

FIG. 9 is a timing waveform diagram illustrating a relationship betweena capacitive element shown in the switching power supply in FIG. 8 andoutput voltage fluctuations;

FIGS. 10A and 10B are timing waveform diagrams for explaining anoperation when a capacitive element 123 of the switching power supply inFIG. 8 is not connected;

FIGS. 11A and 11B are timing waveform diagrams for explaining anoperation when the capacitive element 123 of the switching power supplyin FIG. 8 is connected;

FIGS. 12A and 12B are timing waveform diagrams for explaining therelationship between a control circuit of the switching power supply inFIG. 8 and the output voltage fluctuations;

FIGS. 13A and 13B are explanatory diagrams illustrating a low-passfilter which is configured with a voltage-dividing resistance and thecapacitive element of the switching power supply in FIG. 8 and atransmission function thereof;

FIG. 14 is a circuit diagram illustrating the configuration of theswitching power supply according to a fourth embodiment of the presentinvention;

FIGS. 15A and 15B are timing waveform diagrams for explaining anoperation when a capacitive element 123 of the switching power supply inFIG. 14 is connected;

FIG. 16 is a circuit diagram illustrating the configuration of theswitching power supply according to a fifth embodiment of the presentinvention;

FIGS. 17A and 17B are timing waveform diagrams for explaining anoperation when the capacitive element 123 of the switching power supplyin FIG. 16 is connected;

FIG. 18 is a circuit diagram illustrating the configuration of theswitching power supply according to a sixth embodiment of the presentinvention;

FIGS. 19A and 19B are circuit diagrams illustrating configurations ofcomparators and reference voltages according to embodiments of thepresent invention;

FIG. 20 is a circuit diagram illustrating the configuration of theswitching power supply according to a seventh embodiment of the presentinvention;

FIG. 21 is a timing waveform diagram illustrating a relationship betweena capacitive element shown in the switching power supply in FIG. 20 andoutput voltage fluctuations;

FIG. 22 is an explanatory diagram illustrating a transmission functionof a low-pass filter which is configured with a voltage-dividingresistance and the capacitive element of the switching power supply inFIG. 20;

FIG. 23 is a timing waveform diagram for explaining an operation of anOFF period detection circuit 250 of the switching power supply in FIG.20;

FIG. 24 is a circuit diagram illustrating the configuration of theswitching power supply according to an eighth embodiment of the presentinvention;

FIG. 25 is a circuit diagram illustrating the configuration of theswitching power supply according to a ninth embodiment of the presentinvention;

FIG. 26 is a circuit diagram illustrating the configuration of theswitching power supply according to a tenth embodiment of the presentinvention;

FIGS. 27A and 27B are circuit diagrams illustrating configurations ofcomparators and reference voltages according to embodiments of thepresent invention;

FIG. 28 is a circuit diagram illustrating the configuration of theswitching power supply according to the related art;

FIG. 29 is a circuit diagram illustrating the configuration of theswitching power supply according to an eleventh embodiment of thepresent invention;

FIG. 30 is a circuit diagram illustrating the configuration of theswitching power supply according to a twelfth embodiment of the presentinvention; and

FIG. 31 is a circuit diagram illustrating the configuration of theswitching power supply according to a thirteenth embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Below, preferred embodiments of the present invention will be described.The preferred embodiments of the present invention are not to be limitedto the below-described embodiments. Moreover, the below-describedelements may include what a skilled person may easily arrive at, andwhat may be substantially the same, and those elements may be combinedappropriately.

Embodiments of the present invention will be described in detail withreference to the drawings. In the description of the drawings, the sameletters are given to the same elements, so that repeated explanationsare omitted.

First Embodiment

FIG. 1 is a diagram for explaining a circuit configuration related to aswitching power supply which controls an output voltage by hysteresiscontrol according to a first embodiment of the present invention. Aswitching power supply 1 a shown in FIG. 1 includes a main circuit 4 anda control circuit 9 a.

Moreover, the switching power supply 1 a includes a pair of inputterminals 2 a and 2 b (also called “an input terminal 2” when notspecifically distinguishing therebetween); and a pair of outputterminals 3 a and 3 b (also called “an output terminal 3” when notspecifically distinguishing between);

More specifically, in between the pair of input terminals 2 a and 2 b,an input voltage (DC voltage) V1 is input with the input terminal 2 bwhich is connected to the reference potential (the common ground G inthe present embodiment) as the low potential side. In between the pairof output terminals 3 a and 3 b, an output voltage (DC componentvoltage+output voltage ripple) V2 is output with the output terminal 3 bwhich is connected to the reference potential (the common ground G inthe present example) as the low potential side.

The main circuit 4 is a voltage conversion circuit and is configuredwith a switching device including a switching element which iscontrolled by the controller 5, an input capacitor, a rectificationdevice including a diode and an FET, a smoothing device including achoke coil, a capacitor, etc., for example. The main circuit 4 convertsan input voltage V1 input from the input terminal 2 to an output voltageV2 to output the converted result to the output terminal 3.

The control circuit 9 a includes a controller 5; a level shift circuit10, a comparator 7, an operating power supply 6 for the comparator 7, areference voltage source 8, and a constant voltage source 11, andcontrols a switch operation of the main circuit 4. The comparator 7includes a first input terminal and a second input terminal. The firstinput terminal of the comparator 7 is connected to an output terminal 3a, while the second input terminal is connected to the positive terminalof the reference voltage source 8. The output terminal of the comparator7 is connected to the level shift circuit 10 via an output line whichoutputs an output control pulse Sp1.

Moreover, the comparator 7 includes a negative power supply terminal anda positive power supply terminal. The negative power supply terminal isconnected to the negative terminal of the reference voltage source 8;the positive terminal of the constant voltage source 11; and thenegative terminal of the operating power supply 6. The positive powersupply terminal is connected to the positive terminal of the operatingpower supply 6. In this way, a power supply which operates thecomparator 7 is provided. The negative terminal of the constant powersource 11 and the output terminal 3 b are connected to the common groundG.

As an example of control of the switch element operation, when theoutput Sc of the controller takes an H level, the output voltage V2 ofthe power supply rises. When the output of the controller takes an Llevel, the output voltage V2 of the power supply drops. Below, anoperation of the control circuit 9 a is described in detail withreference to FIGS. 1 and 2.

First, FIG. 1 is described. The output voltage V2 from the outputterminal 3 is input to the first input terminal of the comparator 7. Theinput voltage V2 is the DC output voltage V2, which includes an outputvoltage ripple component. The switching power supply 1 a is configuredin which this output voltage ripple may be regarded as a voltage havingthe same phase as the voltage across the ESR of the output capacitor, orthe converter may be regarded as a constant current source, and the ACcomponent voltage (below also called “an output voltage ripple” when notdistinguishing therebetween) of the voltage across the output capacitoris a triangular wave, or has a waveform which is similar to thetriangular wave. When the output of the comparator 7 is brought to afirst level, the voltage level of an output control pulse Sp1 of thecomparator 7 causes a voltage level Vb of the constant voltage source 11to be output in an approximate manner for the output voltage V2 (seeFIG. 2). This voltage level Vb is input to the level shift circuit 10,which is provided between the comparator 7 and the input of thecontroller 5. Based on the control pulse Sp1, which is output by thecomparator 7, the level shift circuit 10 subtracts the voltage level Vbof the constant voltage source 11 therefrom to shift it to the zerolevel to output an L-level output control pulse Sp2. When this L levelis input to the controller 5, based on the output control pulse Sp2 ofthe level shift circuit 10, the controller 5 stops or turns off a switchoperation of the power supply. Thus, the output voltage V2 of theswitching power supply 1 a drops.

Next, when the output voltage V2 drops to bring the output of thecomparator 7 to the second level, the voltage level of the outputcontrol pulse Sp1 of the comparator 7 causes the voltage level of thesum of the voltage level Vb of the constant voltage source 11 and thevoltage level Vc of the operating power supply 6 (Vb+Vc) to be output inan approximate manner. When this voltage level (Vb+Vc) is input to thelevel shift circuit 10, based on the control pulse Sp1, which is outputby the comparator 7, the level shift circuit 10 subtracts the voltagelevel Vb of the constant voltage source 11 therefrom to shift it to thevoltage level Vc of the operating power supply 6 to cause the shiftedresult to be output from SP2 as the H level. When this H level is inputto the controller 5, the controller 5 starts or turns on the switchoperation of the power supply based on the output control Sp2 of thelevel shift circuit. Thus, the output voltage V2 of the power supplyrises. The operation is repeated in this manner. Needless to say, thefirst level and the second level, which are outputs of the comparator 7,are different levels.

Next, an operation during the period of t0 to t1, or t2 to t3 isdescribed based on the timing waveform diagram in FIG. 2. The outputvoltage V2 during this period rises from a second voltage to a firstvoltage. The voltage level of the output control pulse Sp1 of thecomparator 7 is approximately the voltage level of the sum of thevoltage level Vb of the constant voltage source 11 and the voltage levelVc of the operating power supply 6 (Vb+Vc). The voltage level of theoutput control pulse Sp2 of the level shift circuit 5 causes an H levelto be output.

Therefore, an output of the controller 5 outputs, in this period, adrive signal which turns on a switch of the power supply from thecontroller 5 as a control signal Sc based on the control pulse Sp2.Alternatively, the output of the controller 5, in this period, outputs adrive signal which turns a switch of the main circuit 4 ON/OFF aplurality of times from the controller 5 as a control signal Sc.

Next, an operation during the period of t1 to t2, or t3 to t4 in FIG. 2is described. The output voltage V2 reaches the first voltage at t=t1 ort=t3 to cause the voltage level of the output control pulse Sp1 of thecomparator 7 to approximately switch to the voltage level Vb of theconstant voltage source 11. Thereafter, during the period of t1 to t2,or t3 to t4, this voltage level Vb is maintained. Similarly, the voltagelevel of the output control pulse Sp2 of the level shift circuit 10 isswitched to a L level, and, in the period of t1 to t2, or t3 to t4, thevoltage of this L level is maintained.

The output of the controller 5 outputs, from Sc of the controller 5, adrive signal which turns off a switching element of the main circuit 4based on the control pulse Sp2 of the level shift circuit 10. The outputvoltage V2 of the switching power supply 1 a drops.

At t=t2 or t=t4, the output voltage V2 of the switching power supply 1 areaches the second voltage to switch the voltage level of the outputcontrol pulse Sp1 of the comparator 7 again to approximately the voltagelevel of the sum of the voltage level Vb of the constant voltage source11 and the voltage level Vc of the operating power supply 6. The voltagelevel of the output control pulse Sp2 of the level shift circuit 10switches to the H level.

Based on the output control pulse Sp2 of the level shift circuit 10, theoutput of the controller 5 outputs a drive signal which turns on theswitching element of the main circuit 4 from Sc of the controller 5.Alternatively, based on the output control pulse Sp2 of the level shiftcircuit 10, the output of the controller 5, in this period, outputs adrive signal which turns ON/OFF the switching element of the switchingpower supply 1 a a plurality of times from Sc of the controller 5. Theoperation is repeated in this manner. Needless to say, the first voltageand the second voltage are different voltage values.

Next, one specific embodiment of the control circuit 9 a shown in FIG. 1is described with reference to FIG. 3. The difference between FIG. 1 andFIG. 3 is that the level shift circuit 10 shown in FIG. 1 is shown withspecific circuit features including a first Zener diode 10 b and a firstresistor 10 a and that the constant voltage source 11 is shown withspecific circuit features including a resistor 11 a, a Zener diode 11 b,and a capacitor 11 c. The other features are the same as in FIG. 1.Moreover, the level shift circuit 10 may be configured with the firstresistor 10 a and the first Zener diode 10 b.

The first Zener diode 10 b has the anode connected to one end of thefirst resistor 10 a and the controller 5. Moreover, the cathode of thefirst Zener diode 10 b is connected to the output terminal of thecomparator 7. The other end of the first resistor 10 a is connected tothe common ground G.

When a pulse voltage from the output pulse Sp1 of the comparator 7 isapplied to the cathode of the first Zener diode 10 b, the level shiftcircuit 10 outputs a pulse voltage level of the Sp2 in which the voltagelevel of the output pulse Sp1 of the comparator is level-shifted to theinput of the controller 5 from the anode of the first Zener diode 10 b.

The shift voltage level value of the level shift circuit 10 may be setby the first Zener diode 10 b. The current which flows into the firstZener diode 10 b may be set by the first resistor 10 a.

With the Zener voltage Vz across the first Zener diode 10 b being setequal to the voltage of the constant voltage source 11, the outputcontrol pulse Sp1 of the comparator 7 causes the voltage level Vb of theconstant voltage source 11 to cause the L level to be output from theoutput terminal Sp2 of the level shift circuit 10. With the voltagelevel of the sum of the voltage level Vb of the constant voltage source11 and the voltage level Vc of the operation power supply 6 (Vb+Vc)being output, the output control pulse Sp1 of the comparator 7 causesthe H level to be output from the output terminal Sp2 of the level shiftcircuit. In this way, the level shift circuit 10 shifts the outputvoltage level value from the comparator 7.

Moreover, besides the configurations as described above, theabove-described level shift circuit 10 may also be configured from aninsulating transformer. Alternatively, it may also be configured using acapacitor, a logic circuit such as AND, a flip-flop circuit, a latchcircuit, an RS flip-flop circuit, and a JK flip-flop circuit.Alternatively, it may also be configured using a photo coupler. Thelevel shift circuit does not have to be used as long as the input of thecontroller may recognize the output levels (the first level and thesecond level) of the comparator to correctly control the switchingelement of the switching power supply.

The constant voltage source 11 shown in FIG. 3 includes a secondresistor 11 a, wherein an input voltage V1 is applied to one endthereof; a second Zener diode 11 b which is connected to point A, whichis the other end of the second resistor 11 a; and a first capacitor 11 cwhich is connected to the cathode of the second Zener diode 11 b and oneend of the point A. The anode of the second Zener diode 11 b and theother end side of the first capacitor 11 c are connected to the commonground G. Moreover, the point A is connected to the negative terminal ofthe reference power source 8, the negative terminal of the operatingpower supply 6 of the comparator 7; and the ground terminal of thecomparator 7. Such a configuration makes it possible to create theconstant voltage source 11.

The voltage value of the constant voltage source 11 may be set by thesecond Zener diode 11 b. The current which flows through the secondZener diode 11 b may be set by the second resistor 11 a. The firstcapacitor 11 c is provided to suppress the AC component (the voltageripple component) of the second Zener diode 11 b. The capacitance of thefirst capacitor 11 c is preferably at least 0.01 μF.

The voltage which is applied to the one end of the second resistor 11 ais applied from the input voltage V1 of the switching power supply 1 bas one example. If there is a voltage from a different voltage linewhose level is at least a voltage level required by the constant voltagesource, the voltage may be applied to the one end of the second resistor11 a.

Moreover, besides the configuration as described above, theabove-described constant voltage source 11 may also be configured from acapacitor and a linear regulator to accurately secure a constant voltagesource. It may also be configured from a capacitor and a switchingregulator. Alternatively, it may be configured from a capacitor and acharge-pump circuit.

Second Embodiment

Next, explanations will be given with reference to FIG. 4, which is adiagram of the circuit configuration related to the switching powersupply which controls the output voltage by the hysteresis controlaccording to the second embodiment of the present invention. Thedifference between a switching power supply 1 c shown in FIG. 4 and theswitching power supply 1 a shown in FIG. 1 is that the internalconfiguration of the control circuit 9 differs therebetween. The otheraspects are the same as those in FIG. 1.

A control circuit 9 c includes a controller 5, a comparator 7, anoperating power supply 6 of the comparator 7, a reference voltage source8, and a constant voltage source 12, and controls a switch operation ofthe main circuit 4.

The positive terminal of the constant voltage source 12 is connected tothe output terminal 3 a thereof and the negative terminal is connectedto the first input terminal of the comparator 7. The comparator 7includes a first input terminal and a second input terminal and thesecond input terminal is connected to the positive terminal of thereference voltage source 8. The output terminal of the comparator 7 isconnected to the controller 5 via an output line which outputs an outputcontrol pulse Sp1.

Moreover, the comparator 7 includes a negative power supply terminal anda positive power supply terminal. The negative power supply terminal isa common ground potential which is connected to the negative terminal ofthe reference voltage source 8 and the negative terminal of theoperating source 6. The positive power supply terminal is connected tothe positive terminal of the operating power supply 6. This provides apower supply which operates the comparator 7. As an example, when theoutput Sc of the controller 5 is at the H level, the output voltage V2of the switching power supply 1 c rises. When the output of thecontroller 5 is at the L level, the output voltage V2 of the switchingpower supply 1 c drops. Below, the operation of the control circuit 9 cis described in detail with reference to FIGS. 4 and 5.

First, FIG. 4 is described. The output voltage V2 from the outputterminal 3 is input to the positive terminal of the constant voltagesource 12. A voltage Va in which a voltage Vd across the constantvoltage source 12 is subtracted from the DC voltage V2, which includesthe output voltage ripple component, is output from the negativeterminal of the constant voltage source 12 and input to a first inputterminal of the comparator 7.

The input voltage Va is the DC output voltage Va, which includes theoutput voltage ripple component. The switching power supply 1 c isconfigured in which this output voltage ripple may be regarded as avoltage having the same phase as the voltage across the ESR of theoutput capacitor, or the converter may be regarded as a constant currentsource, and the AC component voltage (below also called “an outputvoltage ripple” when not distinguishing therebetween) of the voltageacross the output capacitor is a triangular wave, or has a waveformwhich is similar to the triangular wave. When the output of thecomparator 7 is brought to a first level, the voltage level of an outputcontrol pulse Sp1 of the comparator 7 causes an L level to be output forthe input voltage Va. When this L level is input to the controller 5,based on the output control pulse Sp1 of the comparator 7, thecontroller 5 stops or turns off a switch operation of the power supply.Thus, the output voltage V2 of the switching power supply 1 c drops.

Next, when the output voltage V2 drops to bring the output of thecomparator 7 to the second level, the voltage level of the outputcontrol pulse Sp1 of the comparator 7 causes an H level to be outputfrom the SP1 as the voltage level of the operating power supply 6 in anapproximate manner. When this H level is input to the controller 5, thecontroller 5 starts or turns on the switching operation of the powersupply based on the output control pulse Sp1 of the comparator 7. Thus,the output voltage V2 of the power supply rises. The operation isrepeated in this manner.

Next, an operation during the period of t0 to t1, or t2 to t3 isdescribed based on the timing waveform diagram in FIG. 5. The outputvoltage V2 during this period rises. The input voltage Va of thecomparator 7 rises by the voltage (V2−Vd). The voltage level of theoutput control pulse Sp1 of the comparator 7 is brought to approximatelythe voltage level Vc of the operating power supply 6 to cause an H levelto be output. The output of the controller 5 outputs, in this period,based on the output control pulse Sp1 of the comparator 7, a drivesignal which turns on a switching element of the switching power supply1 c to be turned on from the controller 5 as a control signal Sc.Alternatively, the output of the controller 5, in this period, based onthe output control pulse Sp1 of the comparator 7, outputs a drive signalwhich turns ON/OFF the switching element of the switching power supply 1c a plurality of times from the controller 5 as the control signal Sc.

Next, an operation during the period of t1 to t2, or t3 to t4 in FIG. 5is described. The input voltage Va reaches the first voltage at t=t1 ort=t3 to cause the voltage level of the output control pulse Sp1 of thecomparator 7 to approximately take the common ground G to switch to theL level. Thereafter, during the period of t2 to t4, the level ismaintained. The output of the controller 5, based on the output controlpulse Sp1 of the comparator 7, outputs a drive signal which turns off aswitching element of the switching power supply 1 c from the controller5 as a control signal Sc. The output voltage V2 in this period drops.

The input voltage Va of the comparator 7 drops by the voltage (V2−Vd).At t=t2 or t=t4, the input voltage Va reaches the second voltage tocause the voltage level of the output control pulse Sp1 of thecomparator 7 to approximately switch again to the voltage level Vc ofthe operating power supply 6. Based on the output control pulse Sp1 ofthe comparator 7, the output of the controller 5 outputs a drive signalwhich turns on the switching element of the switching power supply 1 cas the control signal Sc from the controller 5. Alternatively, based onthe output control pulse Sp1 of the comparator 7, the output of thecontroller 5, in this period, outputs a drive signal which turns ON/OFFthe switching element of the switching power supply 1 c a plurality oftimes as a control signal Sc from the controller 5. The operation isrepeated in this manner.

Next, one example of the control circuit 9 c shown in FIG. 4 isdescribed with reference to FIG. 6. The difference between FIG. 4 andFIG. 6 is that the constant voltage source 12 shown is FIG. 4 is shownwith specific circuit features including a resistor 12 b and a Zenerdiode 12 a. The other features are the same as those in FIG. 4.

The positive terminal of the Zener diode 12 a is connected to the outputterminal 3 a thereof and the negative terminal is connected to the firstinput terminal of the comparator 7. The cathode of the Zener diode 12 ais connected to one end of the resistor 12 b and is brought to an inputvoltage Va. The other end of the resistor 12 b has the common ground Gpotential. In the constant voltage source 12 shown in FIG. 6, when theinput voltage V2 is applied to the cathode of the third Zener diode 12a, the voltage Va is input to a first input terminal of the comparator7.

The voltage value of the constant voltage source 12 may be set by thethird Zener diode 12 a. The current which flows through the constantvoltage source 12 may be set by the resistor 12 b.

The voltage Va, which is input to the first input of the comparator 7,is a voltage (V2−Vd) in which a voltage Vd across the third Zener diode12 a is subtracted from the output voltage V2, and is brought to beequal to the input voltage Va, which is input to the first inputterminal of the comparator 7.

Next, one example of the control circuit 9 c shown in FIG. 4 isdescribed with reference to FIG. 7. The difference between FIG. 4 andFIG. 7 is that the constant voltage source 12 shown is FIG. 4 is shownwith specific circuit features including resistors 12 b, 12 d, 12 e anda shunt regulator 12 c. The other features are the same as those in FIG.4. The constant voltage source circuit 12 shown in FIG. 7 is configuredto include resistors and a three-terminal variable shunt regulator toaccurately secure a constant voltage source.

The cathode of the shunt regulator 12 c is connected to an outputterminal 3 a, while the anode thereof is connected to a first inputterminal of the comparator 7. The first terminal of the comparator 7 isconnected to one end of the resistor 12 b, while the other end of theresistor 12 b is the common ground potential.

When the input voltage V2 is applied to a Cathode terminal 51 of theshunt regulator 12 c, the constant voltage circuit 12 has a voltage Vainput to the first input terminal of the comparator 7 via an Anodeterminal 53 of the shunt regulator.

A resistor 12 d may be connected between the terminal 51 of the shuntregulator and the variable terminal 52 of the shunt regulator and aresistor 12 e may be connected between the variable terminal 52 of theshunt regulator 12 c and the terminal 53 of the shunt regulator toaccurately create the constant voltage source 12.

In other words, setting of the voltage value of the above-describedconstant voltage source 12 may be simply set with values of the resistor12 d and the resistor 12 e. The resistor 12 b makes it possible to setthe current which flows through the shunt regulator 12 c and theresistor 12 d and the resistor 12 e.

Moreover, besides the configurations as described above, the constantvoltage source 12 may be configured with an insulating transformer.Alternatively, it may also be configured with a capacitor and a linearregulator. Alternatively, it may be also be configured with a capacitorand a switching regulator. Alternatively, it may be also be configuredwith a capacitor and a charge-pump circuit.

For the above-described comparator 7, a comparator having hysteresisproperties may be used. Moreover, the above-described comparator 7 mayalso be used for a window comparator, etc., for example, that allowsproducing the first and second voltages.

Third Embodiment

FIG. 8 is a circuit diagram illustrating a configuration of a switchingpower supply 101 a according to a third embodiment of the presentinvention. As one example, the switching power supply 101 a shown inFIG. 8 includes a pair of input terminals 102 a and 102 b (also called“an input terminal 102” when not specifically distinguishingtherebetween); a pair of output terminals 103 a and 103 b (also called“an output terminal 103” when not specifically distinguishing between);a main circuit 104 a; and a control circuit 120 a, and converts an inputvoltage (DC voltage) V1 input to the input terminal 102 to an outputvoltage (DC voltage) V2 to output the converted result from the outputterminal 103 and controls the output voltage V2 to be at a predeterminedtarget voltage. The switching power supply 101 a inputs an input voltageV1 and an input current i1 to the input terminal 102 to output theoutput voltage V2 and a load current i2 from the output terminal 103.

The main circuit 104 a includes a switching transistor 105 a, a parasitediode 105 b of the switching transistor 105 a, a diode 106, a choke coil107, an output capacitor 108 a, and an equivalent series resistor 108 bof the output capacitor 108 a. The switching power supply 101 a, whichis configured with a back converter circuit scheme as one example,outputs an input voltage V1 input from the input terminal 102 to theoutput voltage V2 to output the converted result to the output terminal103.

The control circuit 120 a includes a third resistor 121 and a fourthresistor 122 that voltage-divides the output voltage V2 of the switchingpower supply 101 a a comparator 125 a in which a voltage-divided voltageVn is input to a first inverting input terminal thereof and a referencevoltage Vp is input to a second non-inverting input terminal thereof;and a controller 130 which controls a switching transistor 105 a basedon an output signal Vco of the comparator 125 a, wherein the referencevoltage Vp is brought to be a first high VpH when an output Vco of thecomparator 125 a takes a first high voltage VpH and is brought to be asecond low VpL when the output Vco of the comparator 125 a takes asecond low voltage VpL. The control circuit 120 a includes the thirdresistor 121, which is connected between a positive terminal 103 a of anoutput terminal and a first inverting input terminal of the comparator125 a; and a capacitive element 123 which is connected in parallel withthe third resistor 121. A common ground G of the control circuit 120 ais connected to a negative terminal 103 b of the output terminal. Eachof signal voltages Vn, Vp, and Vco is set to be a voltage with thecommon ground G as the reference.

Moreover, a so-called hysteresis comparator is known in which areference voltage Vp is brought to be a first high voltage VpH when anoutput Vco of the comparator 125 a takes a first high level and anoutput Vco of the comparator 125 a takes a second low level. Theswitching power supply 101 a is configured with a hysteresis comparatorcircuit scheme as an example and includes the comparator 125 a; aresistor 125 b which connects between a non-inverting input terminal andan output terminal of the comparator 125 a; and a resistor 125 c and aconstant voltage source 124 that are connected in series between thenon-inverting input terminal and the common ground G.

Next, an operation of the control circuit 120 a is described. When thevoltage Vn in which the output voltage V2 is resistance-divideddecreases below the second voltage VpL, the comparator output Vco isbrought to a first high level to cause the controller 130 to startdriving the switching transistor 105 a, so that the reference voltage isbrought to be the first voltage VpH. During the drive period of theswitching transistor 105 a, the current of the choke coil 107 increasesto cause a current iL which is larger than a load current i2 to besupplied from the choke coil 107 to the output capacitor 108 a, causingthe output capacitor 108 a to be charged to raise the output voltage V2.When the output voltage Vn in which the voltage V2 is resistance-dividedis brought to be higher than the first voltage VpH, the comparatoroutput Vco is brought to a second low level to cause the drive period tobe completed and to be transitioned to a stop period. At this time, thereference voltage is brought to be the second voltage VpL. In the stopperiod, the load current i2 is brought to be larger than the current iLfrom the choke coil 107 to cause discharging from the output capacitor108 a to cause the output voltage V2 to decrease. When the voltage Vn inwhich the output voltage V2 is resistance-divided is again brought to belower than the second voltage VpL, the drive period is started again tobring the reference voltage to the first voltage VpH. Such an operationis repeated to cause the drive period and the stop period to becontrolled such that the resistance-divided voltage Vn is brought to bea value between the first voltage VpH and the second voltage VpL tocause the output voltage V2 to be controlled to the predetermined targetvoltage.

Here, when the resistance value of the third resistor 121 is set to R1,the resistance value of the fourth resistor 122 is set to R2, theminimum switching frequency of the switching transistor 105 a is set toFmin, and the static capacitance is set to C1, the capacitive element123, which is connected in parallel with the third resistor 121, meetsEquation (1)

$\begin{matrix}{{C\; 1} \geqq \frac{{R\; 1} + {R\; 2}}{2\pi \; F_{m\; i\; n}R\; 2^{2}}} & (1)\end{matrix}$

This makes it possible to suppress static load fluctuations of theoutput voltage when the load current i2 decreases to be a minimumswitching frequency Fmin to around the difference between the firstvoltage and the second voltage. Equation (1) is described below.

Next, an effect of suppressing the output voltage fluctuations by thecapacitive element 123, which is connected to the third resistor 121, isdescribed using a waveform of the output voltage V2. FIG. 9 is a timingwaveform diagram illustrating a relationship between the capacitiveelement 123 and the output voltage fluctuations. The waveform of theoutput voltage V2, which is schematically shown in FIG. 9, is a waveformwhich is extended in the Y-axis direction except for the DC component ofthe output voltage V2. The X axis shows time, while the Y axis shows avoltage ripple of the output voltage. Moreover, for the staticcapacitance C1 of the capacitive element 123, a voltage ripple of theoutput voltage V2 and static load fluctuations are shown, in FIG. 9, forcases of (a) in which there is no capacitive element; (b) in which thestatic capacitance C1 which is beyond the range of the staticcapacitance determined with Equation (1) is connected; and (c) in whichthe static capacitance C1 which meets the static capacitance determinedwith Equation (1) is connected. Moreover, in FIG. 9, each of (a), (b),and (c) shows cases in which the load is a rated load and in which theload is a light load.

In a case without a capacitive element that is shown in (a) in FIG. 9,the voltage ripple of the output voltage V2 is increased up to theproduct of a voltage-dividing ratio (R1+R2)/R2 and the differenceVpH−VpL between the first voltage and the second voltage. There are nochanges in the voltage ripple regardless of whether the load connectedto the output terminal 103 is a rated load or a light load, and, for thelight load, the switching frequency is lower and the stop period islonger.

In a case in which a static capacitance C1 which is smaller than astatic capacitance determined by Equation (1) shown in (b) in FIG. 9 isconnected, the voltage ripple of the output voltage V2 may be decreasedto around the difference VpH−VpL between the first voltage and thesecond voltage. Regardless of whether the load connected to the outputterminal 103 is the rated load or the light load, the voltage ripple maybe decreased to around the same level. However, the average value of theoutput voltage V2 differs between the rated load and the light load, sothat there are large static load fluctuations.

In a case in which a static capacitance C1 which is larger than a staticcapacitance determined by Equation (1) shown in (c) in FIG. 9 isconnected, the static load fluctuations, which represent a differencebetween the light load and the rated load of the average value of theoutput voltage V2 in addition to the voltage ripple of the outputvoltage V2 also may be suppressed to around the difference VpH−VpL ofthe first voltage and the second voltage. Therefore, the output voltagefluctuations may be suppressed.

Next, the reason that the voltage ripple of the output voltage V2 isbrought to the product of the voltage-dividing ratio (R1+R2)/R2 and thedifference VpH−VpL between the first voltage and the second voltage whenthe capacitive element 123 is not connected is described. The outputvoltage V2 is voltage-divided between the third resistor 121 and thefourth resistor 122, so that the voltage Vn, which is input to theinverting input terminal of the comparator 125 a, is voltage-divided tothe product of R2/(R1+R2) and the output voltage V2. In this way, thevoltage ripple of Vn is also attenuated to the product of R2/(R1+R2) andthe voltage ripple of the output voltage V2. The control circuit 120 acontrols the drive period and the stop period such that Vn is brought toa value between VpL and VpH to control the output voltage V2 to apredetermined target voltage. Therefore, the voltage ripple of Vn isbrought to be equal to the difference VpH−VpL of the first voltage andthe second voltage to cause the voltage ripple of the output voltage V2to increase up to the product of (R1+R2)/R2 and the voltage ripple ofVn, so that the magnitude of the voltage ripple of the output voltage V2is brought to be (VpH−VpL)(R1+R2)/R2.

Next, the reason is described that the capacitive element 123 may beconnected to decrease the voltage ripple of the output voltage V2 fromthe product of (R1+R2)/R2 and the difference VpH−VpL of the firstvoltage and the second voltage to around the difference VpH−VpL of thefirst voltage and the second voltage. The capacitive element 123 isconnected to the third resistor 121 to cause the voltage Vr1 of thethird resistor 121 to be stabilized to almost a value. In this way, thevoltage Vn of the inverting input terminal of the comparator 125 a isbrought to a value in which a Vr1 is subtracted from the output voltageV2 to cause the voltage ripple of Vn to be equal to the voltage rippleof the output voltage V2. The control circuit controls the drive periodand the stop period such that Vn is brought to be a value between VpLand VpH to control the output voltage V2 to a predetermined targetvoltage. Therefore, the voltage ripple of Vn is brought to be equal tothe difference VpH−VpL between the first voltage and the second voltageto also bring the magnitude of the voltage ripple of the output voltageV2 to VpH−VpL.

As V2=Vn+Vr1, the output voltage V2 is controlled to a value betweenVpH+Vr1 and VpL+Vr1.

However, the voltage ripple of the output voltage V2 is brought to alittle larger than (VpH−VpL)(R1+R2)/R2 in the case without thecapacitive element 123 as shown in (a) in FIG. 9, and is brought to be alittle larger than VpH−VpL in the case of connecting the capacitiveelement 123 as shown in (b) and (c) in FIG. 9. The reason is due to thefact that the output voltage V2 reaches the maximum, before it starts todrop, later than the completion of the drive period of the switchingtransistor 105 a and the output voltage V2 reaches the minimum, beforeit starts to rise, later than the completion of the stop period of theswitching transistor 105 a. This is described using the ripple waveformof the output voltage V2 in FIGS. 10A and 10B.

FIG. 10A is an operation waveform when operating in a current continuousmode in which a coil current iL of the switching power supply 101 a iscontinuously changed and FIG. 10B is an operation waveform whenoperating in a current non-continuous mode in which the coil current iLof the switching power supply 101 a is non-continuously changed.

In FIGS. 10A and 10B, i2 is a load current, iL is a current of the chokecoil 107, Vco is an output of the comparator 125 a, 108 a is a voltageof the output capacitor 108 a, 108 b is a voltage of 108 b of an ESR ofthe output capacitor, and V2 is an output voltage.

As shown in FIG. 10A, the current iL of the choke coil 107 increaseswhen the output Vco of the comparator 125 a takes a high level (firstlevel) in the period of t0-t2, and, decreases when it takes a low level(second level) in the period of t2-t4. For the load current i2, a DCcurrent which is an average of iL flows. The voltage of the outputcapacitor 108 increases when iL>i2 in the period of t1-t3 and decreaseswhen iL<i2 in the period of t0-t1, and t3-t4. The voltage of the ESR(Equivalent Series Resistance) of the output capacitor 108 b takes avalue in which iL−i2 is multiplied by a resistance value, so that itincreases simultaneously with an increase in iL and decreasessimultaneously with an decrease in iL. The output voltage V2 is broughtto a sum of the voltage of the output capacitor 108 a and the voltage ofthe ESR 108 b of the output capacitor,

In the case without connecting the capacitive element 123, the outputVco of the comparator 125 a is brought to the low level when the outputvoltage V2 is brought to be larger than VpH(R1+R2)/R2 in t2 and theoutput Vco of the comparator 125 a is brought to the high level when theoutput voltage V2 is brought to be smaller than VpL (R1+R2)/R2 in t0 ort4. The output voltage V2 is a sum of the voltage of the outputcapacitor 108 a and the voltage of the ESR 108 b of the outputcapacitor, so that, with a delay after the output Vco of the comparator125 a is brought to the low level in t2, the value of the output voltageV2 increases to be larger than VpH (R1+R2)/R2, after which it reachesthe maximum in t3 and then starts to decrease, and, with a delay afterthe output Vco of the comparator 125 a is brought to the high level int0, the value of the output voltage V2 decreases to be lower than VpL(R1+R2)/R2, after which it reaches the minimum in t1 and then starts toincrease. Then, in the case without the capacitive element 123, thevoltage ripple of the output voltage V2 is brought to be somewhat largerthan (VpH−VpL)(R1+R2).

As shown in FIG. 11A, also for the same reason, in the case in which thecapacitive element 123 is connected in (b) and (c) in FIG. 9, itincreases to be somewhat larger than VpH−VpL. As the output voltage V2is a sum of the voltage of the output capacitor 108 a and the voltage ofthe ESR 108 b of the output capacitor, with a delay after the output Vcoof the comparator 125 a is brought to a low level, the value of theoutput voltage V2 increases to be larger than VpH+Vr1, after which itreaches the maximum and then starts to decrease, and, with a delay afterthe output Vco of the comparator 125 a is brought to a high level, thevalue of the output voltage V2 decreases to be smaller than VpL+Vr1,after which it reaches the minimum and then starts to increase.

Even when operating in the current non-continuous mode of FIGS. 10B and11B, in the same manner as in the current continuous mode, the voltageripple of the output voltage V2 is brought to be somewhat larger. Thedifference from the case of the current continuous mode is that theoutput of the comparator 125 a is brought to a low level in t6, afterwhich the current iL of the choke coil 107 is decreased to be 0 in t7,and a period in which the current iL of the choke coil 107 is 0continues in the period between t7 and t8. In the period in which thecurrent iL of the choke coil 107 is 0, the load current i2 is dischargedfrom the output capacitor 108 a to cause the output voltage V2 to droplinearly. More specifically, when the load current i2 is small with thelight load, the drop of the output voltage V2 is slow, so that it takestime from when the voltage Vn of the inverting input terminal of thecomparator 125 a reaches VpL to when the output of the comparator 125 ais brought to the high level, causing the stop period to be long and theswitching frequency to decrease.

Next, causes for the average value of the output voltage V2 for thelight load being lower than that for the rated load for connecting astatic capacitance which is smaller than the static capacitancedetermined with Equation (1) in (b) in FIG. 9 are described using FIGS.12A and 12B.

FIG. 12A is a timing waveform diagram for connecting a staticcapacitance C1 which is smaller than the static capacitance determinedwith Equation (1), while FIG. 12B is a timing waveform diagram forconnecting the static capacitance C1, which is substantially larger thanthe static capacitance determined with Equation (1). In FIGS. 12A and12B, Vco is an output of the comparator 125 a; V2 is an output voltage,Vr1 is a voltage of the voltage-dividing resistor 121; Vn is aninverting input terminal voltage of the comparator 125 a; Vn_AVG is theaverage value of Vn; ir1 is a current of the voltage-dividing resistor121; and ir2 is a current of the voltage-dividing resistor 122.

FIG. 12A shows the state of the control circuit 120 a when operated withthe light load and the current non-continuous mode. The comparator 125 ahas a short period of a high output level and a long period of a lowoutput level, causing the waveform of the output voltage V2 to have ashort period in which the voltage increases and to have a long period inwhich the voltage decreases with a triangular wave in which the voltagelinearly decreases.

FIG. 13A shows a low-pass filter which has V2 as an input and Vr1 as anoutput with voltage-dividing resistors 121 and 122, and the capacitiveelement 123, while FIG. 13B shows the relationship between the frequencyand the gain of the low-pass filter shown in FIG. 13A. The transmissionfunction of the low-pass filter is to take the following Equation (2):

$\begin{matrix}{\frac{\Delta \; V_{r\; 1}}{\Delta \; V_{2}} = \frac{\frac{1}{C\; 1R\; 1}}{s + \frac{{R\; 1} + {R\; 2}}{C\; 1R\; 1R\; 2}}} & (2)\end{matrix}$

The voltage ripple of V2 at the low frequency with the light load maynot be sufficiently attenuated by the low-pass filter, so that thevoltage ripple remains with Vr1 as shown in FIG. 12A. The invertinginput terminal voltage Vn of the comparator 125 a is a subtraction ofV2−Vr1, so that the waveform of Vn decreases rapidly in the initialperiod in which the voltage decreases, and then the waveform of Vngradually decreases. Therefore, Vn_AVG, which is the average value ofVn, is brought to be a value which is lower than (VpH+VpL)/2, which isthe intermediate value of VpH and VpL. A decrease amount ΔVn from theintermediate value of Vn_AVG is denoted as follows:

$\begin{matrix}{{\Delta \; V_{n}} = {\frac{V_{p\; H} + V_{p\; L}}{2} - V_{n_{AVG}}}} & (3)\end{matrix}$

The current ir2 of the voltage-dividing resistor 122 has a value inwhich Vn is divided by R2, taking the waveform which is similar to thatof Vn as shown. When Vr1 is in the steady state in which Vr1 has thevoltage ripple, the current ir1 of the voltage-dividing resistor 121 isequal to the average of the current ir2 of the voltage-dividing resistor122 and a current ir2−ir1 which flows through the capacitive element 123takes 0, which is the total in one switching period. Thus, the positionrelationship between ir1 and ir2 is similar to the position relationshipbetween Vn_AVG and Vn, causing ir1 to take a value which is lower thanthe intermediate value of ir2 at the same ratio therewith, so that thefollowing relationships in Equations (4) and (5) hold:

V _(n) =i _(r2) R2  (4)

V _(n) _(_) _(AVG) =i _(r1) R2  (5)

ir1 is a current which flows through the resistor 121, causing theaverage value of Vr1 to take the following Equation (6):

$\begin{matrix}{V_{r\; 1} = {V_{n\; \_ \; {AVG}}\frac{R\; 1}{R\; 2}}} & (6)\end{matrix}$

The average value of V2, V2_AVG, is to take the following Equation (7):

$\begin{matrix}{V_{2\; \_ \; {AVG}} = {{V_{n\; \_ \; {AVG}} + V_{r\; 1}} = {V_{n\; \_ \; {AVG}}\frac{{R\; 1} + {R\; 2}}{R\; 2}}}} & (7)\end{matrix}$

On the other hand, for connecting the static capacitance, which issubstantially larger than the static capacitance determined in Equation(1), or for a case in which the relationship in the following Equation(8) holds since the heavy load causes the switching frequency F to besubstantially higher than Fmin for the light load, the waveform in FIG.12B is taken:

$\begin{matrix}{{{C\; 1}}\frac{{R\; 1} + {R\; 2}}{2\pi \; {F \cdot R}\; 2^{2}}} & (8)\end{matrix}$

In the low-pass filter having the V2 as the input and Vr1 as the outputwith the voltage-dividing resistors 121 and 122 and the capacitiveelement 123, the voltage ripple of the output voltage V2 issubstantially attenuated, causing Vr1 to take a waveform which is closeto direct current. The inverting input terminal voltage Vn of thecomparator 125 a is a subtraction of V2−Vr1, where the direct currentVr1 is subtracted from V2 in the triangular wave, resulting in Vn totake the triangular wave. Therefore, as shown in Equation (9) asfollows, the average value Vn_AVG, which is the average value of Vn, isbrought to be almost equal to (VpH+VpL)/2, which is an intermediatevalue of VpH and VpL:

$\begin{matrix}{V_{n\; \_ \; {AVG}} \approx \frac{V_{p\; H} + V_{p\; L}}{2}} & (9)\end{matrix}$

Since Equations (4) to (7) also hold in FIG. 12B, the average valueV2_AVG of V2 in FIG. 12B is to take the following Equation (10):

$\begin{matrix}{V_{2{\_ AVG}} = {\frac{V_{pH} + V_{pL}}{2}\frac{{R\; 1} + {R\; 2}}{R\; 2}}} & (10)\end{matrix}$

The difference ΔV2L of the average value of V2 in FIG. 12A and theaverage value of V2 in FIG. 12B is a static load fluctuation forconnecting the static capacitance C1, which is outside the range of thestatic capacitance determined in Equation (1), which is the differencebetween the average value of V2 in the switching frequency Fmin for thelight load in FIG. 12A and the average value of V2 in the case in whichEquation (8) in met with the switching frequency F for the heavy load inFIG. 12B. The difference ΔV2L of the average value of V2 is given by thefollowing Equation (11) from Equations (3), (7), and (10):

$\begin{matrix}{{\Delta \; V_{2L}} = {\Delta \; V_{n}\frac{{R\; 1} + {R\; 2}}{R\; 2}}} & (11)\end{matrix}$

The decrease ΔVn in the average value of Vn occurs due to subtractingVr1 with the voltage ripple from the triangular wave in V2, so that ΔVnhas a magnitude which is almost the same as the voltage ripple ΔVr1 ofVr1. Therefore, the following Equation (12) holds:

$\begin{matrix}{{\Delta \; V_{2L}} = {\Delta \; V_{r\; 1}\frac{{R\; 1} + {R\; 2}}{R\; 2}}} & (12)\end{matrix}$

As shown in Equation (12), the static load fluctuation ΔV2L of theoutput voltage takes the magnitude which is the product of (R1+R2)/R2and the ripple ΔVr1 of the inter-terminal voltage of R1 with which thecapacitive element 123 is connected in parallel. The output voltage V2being substantially higher than the reference voltage Vp causes thevalue of (R1+R2)/R2 to be large, causing a larger static loadfluctuation.

Next, the fact is explained that the static capacitance C1, which meetsthe static capacitance determined in Equation (1) as the capacitiveelement 123, may be connected to suppress the static load fluctuationΔV2L of the output voltage to around VpH−VpL, which is the differencebetween the first voltage and the second voltage.

As an example of specification of the output voltage V2 of the switchingpower supply 1 a, the output voltage ripple ΔV2 is 1% of the outputvoltage V2 and the total fluctuation of the output voltage v2 is 5% ofthe output voltage, so that the total fluctuation often allows a rangewhich is wider than that allowed by the output voltage ripple. The totalfluctuation here includes a static load fluctuation, a static inputfluctuation, an ambient temperature fluctuation, and a temporal drift,so that 1% or below is desirable for only the static load fluctuation.Therefore, targeting that the static load fluctuation ΔV2L is suppressedto be equal to or below the output voltage ripple ΔV2 results in:

ΔV _(2L) <ΔV ₂  (13)

As the output voltage ripple ΔV2 is suppressed to around VpH−VpL, whichis the difference between the first voltage and the second voltage byproviding the capacitive element 123, ΔV2L<ΔV2 may be achieved to alsosuppress the static load fluctuation ΔV2L of the output voltage toaround VpH−VpL, which is the difference between the first voltage andthe second voltage.

To meet Equation (13), Equation (12) may be substituted into Equation(2) to determine the condition for which the absolute value of thetransmission function shown in Equations (14)(15) is less than or equalto 1:

$\begin{matrix}{\frac{\Delta \; V_{2L}}{\Delta \; V_{2}} = {\frac{{R\; 1} + {R\; 2}}{R\; 2}\frac{\frac{1}{C\; 1R\; 2}}{s + \frac{{R\; 1} + {R\; 2}}{C\; 1R\; 1R\; 2}}}} & (14) \\{s = {j\; 2\pi \; F_{\min}}} & (15)\end{matrix}$

Determining this condition results in the following Equation (16):

$\begin{matrix}{{C\; 1} > {\frac{{R\; 1} + {R\; 2}}{2\pi \; F_{\min}R\; 2^{2}}\sqrt{1 - \left( \frac{R\; 2}{R\; 1} \right)^{2}}}} & (16)\end{matrix}$

The output voltage V2 is substantially larger than the reference voltageVp, so that the voltage-dividing ratio is large; when R1>R2,

$\begin{matrix}{\sqrt{1 - \left( \frac{R\; 2}{R\; 1} \right)^{2}}\overset{.}{=}{.1}} & (17)\end{matrix}$

ignoring the minute term of (R2/R1)², results in C1>(R1+R2)/(2πFmin×R22)in Equation (1).

In FIGS. 13A and 13B are shown, with slanted lines, a range of theswitching frequency in which ΔV2L, which is the static load fluctuationof the output voltage, may also be suppressed to around VpH−VpL, whichis the difference between the first voltage and the second voltage, whenthe low-pass filter with V2 as the input and Vr1 as the output with thevoltage-dividing resistances 121 and 122 and the capacitive element 123is provided. When the switching frequency is in a region which is higherthan the slanted lines for the conditions of heavy load and light load,the static load fluctuation may be suppressed.

The cutoff frequency Fc, at which the gain of the low-pass filterdecreases by 3 dB, is given by the following Equation (18):

$\begin{matrix}{F_{C} = \frac{{R\; 1} + {R\; 2}}{2{\pi C}\; 1R\; 1R\; 2}} & (18)\end{matrix}$

From Equations (18) and (1), the minimum switching frequency Fmin isgiven by the following Equation (19):

$\begin{matrix}{F_{\min} = {\frac{{R\; 1} + {R\; 2}}{{2\pi \; C\; 1R\; 2^{2}}\;} = {\frac{R\; 1}{R\; 2}F_{C}}}} & (19)\end{matrix}$

Fmin needs to be set to the product of R1/R2 and Fc to attenuate thegain of the low-pass filter down to R2/(R1+R2) times. When the outputvoltage V2 is substantially higher than the reference voltage Vp, R1/R2is large and R2/(R1+R2) is small, so that it is necessary to use theswitching frequency in a region which is sufficiently separated from thecutoff frequency Fc and in which attenuation is sufficient.

As an example, for controlling the output voltage V2 to 20V, the firstvoltage VpH of the reference voltage Vp is set to 1.55V the second lowvoltage VpL is set to 1.45V, and the hysteresis width, which is thedifference between the first voltage VpH and the second voltage VpL, isset to 0.1V. The output voltage V2 is 13 times the reference voltage Vp,so that the resistance value R1 of the third resistor 121 is set to be12 times the resistance value R2 of the second resistor R2 tovoltage-divide the output voltage V2. For the target specification ofthe output voltage V2, the output voltage ripple ΔV2 is set to 1% of theoutput voltage V2 and the static load fluctuation ΔV2L is also set to 1%of the output voltage V2.

When the capacitive element 123 is not connected, then the voltage Vn,with the output voltage V2 being 1/13^(th) of the voltage-dividingratio, is input to the inverting input terminal of the comparator 125 a,so that the control circuit controls the output voltage V2 such that Vnis brought to a value between VpL and VpH. Then, the voltage ripple ofVn is brought to be equal to 0.1V of the hysteresis width, which is thedifference of the first voltage VpH and the second voltage VpL, and thevoltage ripple of the output voltage V2 is brought to 1.3V, which is 13times the voltage-dividing ratio of the hysteresis width. Then, theoutput voltage ripple ΔV2 is 6.5% of the output voltage V2, which doesnot meet the specification.

The capacitive element 123 may be connected to decrease the voltageripple of the output voltage V2. If the static capacitance of thecapacitive element 123 is set to 1 nF, the voltage ripple of the outputvoltage V2 decreases to 0.15V. However, when the load connected to theoutput terminal 103 is brought to 10 kΩ to cause the load current todecrease down to 2 mA, the static capacitance of the capacitive element123 is smaller than the static capacitance 100 nF determined in Equation(1) since the minimum switching frequency Fmin is 1.9 kHz, causing theoutput voltage V2 to be lower by around 0.7 V than a case of taking therated load in which the load connected to the output terminal 103 is 3Ωto bring the load current to 7 A. While the voltage ripple ΔV2 of theoutput voltage V2 decreases down to 0.15V (0.75% of the output voltageV2), the static load fluctuation is 0.7V (3.5% of the output voltageV2), so that the specification of the static load fluctuation is notmet.

When the static capacitance of the capacitive element 123 is increasedup to the static capacitance of 100 nF, which is determined in Equation(1), the voltage ripple of the output voltage V2 is brought to 0.15V(0.75% of the output voltage V2), and the static load fluctuation, whichis the difference between the output voltage V2 for the rated load ofthe load current of 7 A and that for the load current of 2 mA, isbrought to 0.1V (0.5% of the output voltage V2), which is almost equalto 0.1V, which is the hysteresis width. In this case, both the outputvoltage ripple and the static load fluctuation meet the specification.The static capacitance of the capacitive element 123 may be increasedbeyond the static capacitance of 100 nF, which is determined in Equation(1) to further increase the static load fluctuation.

As described above, the control circuit according to the presentembodiment includes third and fourth resistors 121 and 122 thatvoltage-divide the output voltage of the switching power supply 101 a; acomparator 125 a in which a reference voltage Vp is input to a secondinput terminal thereof; and a controller 130 which controls a switchingtransistor 105 a based on an output signal of the comparator 125 a,wherein the reference voltage Vp is brought to a first voltage when theoutput of the comparator 125 a takes a first level (high level) and isbrought to a second voltage when the output of the comparator 125 atakes a second level (low level), the third resistor 121 is connectedbetween the output terminal positive terminal 103 a of the switchingpower supply 101 a and the first input terminal of the comparator 125 a;the control circuit includes a capacitive element 123 which is connectedin parallel with the third resistor 121; and the Equation (1) is metwhen the resistance value of the third resistor 121 is set to R1, theresistance value of the fourth resistor 122 is set to R2, the minimumswitching frequency of the switching transistor is set to Fmin, and thestatic capacitance of the capacitive element 23 is set to C1.

In this way, the static load fluctuation of the output voltage when theload current decreases to reach the minimum switching frequency Fmin maybe suppressed to around the difference between the first voltage and thesecond voltage.

Fourth Embodiment

FIG. 14 is a circuit diagram illustrating the configuration of aswitching power supply 101 b according to a fourth embodiment of thepresent invention. The switching power supply 101 b shown in FIG. 14includes a pair of input terminals 102 a and 102 b (also called “aninput terminal 102” when not specifically distinguishing therebetween);a pair of output terminals 103 a and 103 b (also called “an outputterminal 103” when not specifically distinguishing between); a maincircuit 104 b; and a control circuit 120 b, and converts an inputvoltage (DC voltage) V1 input to the input terminal 102 to an outputvoltage (DC voltage) V2 to output the converted result from the outputterminal 103 and controls the output voltage V2 to be at a predeterminedtarget voltage. The switching power supply 101 b inputs an input voltageV1 and an input current i1 to the input terminal 102 to output theoutput voltage V2 and a load current i2 from the output terminal 103.

In the same manner as the main circuit 104 a of the first embodiment,the main circuit 104 b is configured with a back-converter circuitscheme as one example of the switching power supply 101 b and convertsan input voltage V1 input from the input terminal 102 to the outputvoltage V2 to output the converted result to the output terminal 3.

The control circuit 120 b includes a third resistor 121 and a fourthresistor 122 that voltage-divides the output voltage V2 of the switchingpower supply 101 b; a comparator 125 a in which a voltage-dividedvoltage Vn is input to a first inverting input terminal thereof and areference voltage Vp is input to a second non-inverting input terminalthereof; and a controller 130 x which controls a switching transistor105 a based on an output signal Vco of the comparator 125 a, wherein thereference voltage Vp is brought to be a first high VpH when an outputVco of the comparator 125 a takes a first high level and is brought tobe a second low VpL when the output Vco of the comparator 125 a takes asecond low level. The control circuit 120 b includes the third resistor121, which is connected between a positive terminal 103 a of an outputterminal and a first inverting input terminal of the comparator 125 a;and a capacitive element 123 which is connected in parallel with thethird resistor 121. A common ground G of the control circuit 120 b isconnected to a negative terminal 103 b of the output terminal. Each ofsignal voltages Vn, Vp, and Vco is set to be a voltage with the commonground G as the reference.

Moreover, in the same manner as the third embodiment, a so-calledhysteresis comparator is provided in which a reference voltage Vp isbrought to be a first high voltage VpH when an output Vco of thecomparator 125 a takes a first high level and to be a second low voltageVpL when an output Vco of the comparator 125 a takes a second low level.As an example of this hysteresis comparator circuit scheme, in the samemanner as the third embodiment, the switching power supply 101 bincludes the comparator 125 a; a resistor 125 b which connects between anon-inverting input terminal and an output terminal of the comparator125 a; and a resistor 125 c and a constant voltage source 124 that areconnected in series between the non-inverting input terminal and thecommon ground G.

The controller 130 x includes a current detection element 130 d whichdetects a current iL which flows through a choke coil 107; a referencevoltage 130 c; a comparator 130 b which compares an output of thecurrent detection element 130 d and the reference voltage 130 c; and adrive device 130 a which drives the switching transistor 105 a based onan output Vco of the comparator 125 a and an output of the comparator130 b.

The drive device 130 a turns on the switching transistor 105 a when theoutput Vco of the comparator 125 a takes a first high level and theoutput of the comparator 130 b takes a low level. The output of thecomparator 130 b is brought to a high level when turning on theswitching transistor 105 a brings the current iL which flows through thechoke coil 107 to at least a value. Then, when the output Vco of thecomparator 125 a continues at the first high level, the switchingtransistor 105 a is turned off for a period. Turning off the switchingtransistor 105 a causes the current iL which flows though the choke coil107 to be lower than the value. When the output Vco of the comparator125 a continues to be at the first high level even thereafter, theswitching transistor 105 a is turned on until the current iL which flowsthrough the choke coil 107 reaches at least the value again. When theoutput Vco of the comparator 125 a takes a second low level, theswitching transistor 105 a is turned off regardless of the output of thecomparator 130 b.

This causes switching to be performed a plurality of times during theperiod in which the output Vco of the comparator 125 a is at the firsthigh level.

Next, an operation of the control circuit 120 b is described. When thevoltage Vn in which the output voltage V2 is resistance-divideddecreases below the second voltage VpL, the comparator output Vco isbrought to a first high level to cause the controller 130 x to startdriving the switching transistor 105 a, so that the reference voltage isbrought to be the first voltage VpH. During the drive period of theswitching transistor 105 a, the current of the choke coil 107 increasesto cause a current iL which is larger than a load current i2 to besupplied from the choke coil 107 to the output capacitor 108 a, causingthe output capacitor 108 a to be charged to raise the output voltage V2.The current iL which flows through the choke coil 107 being brought tobe at least a value which is larger than the load current turns off theswitching transistor 105 a for a period. Turning off the switchingtransistor 105 a causes the current iL which flows through the chokecoil 107 to be lower than a value. When the output Vco of the comparator125 a continues at the first high level even thereafter, the switchingtransistor 105 a is turned on until the current iL which flows throughthe choke coil 107 reaches at least a value again. This causes switchingto be performed a plurality of times during a period in which the outputVco of the comparator 125 a continues at the first high level. In thecontroller 130 x, when the voltage Vn in which the output voltage V2 isresistance-divided is brought to be higher than the first voltage VpH,the comparator output Vco is brought to a second low level to cause thedrive period to be completed and to be transitioned to a stop period. Atthis time, the reference voltage is brought to the second voltage VpL.In the stop period, the current iL from the choke coil 7 is brought to 0to cause discharging from the output capacitor 108 a by the load currenti2 to cause the output voltage V2 to decrease. When the voltage Vn inwhich the output voltage V2 is resistance-divided is again brought to belower than the second voltage VpL, the drive period is started again tocause the reference voltage to be brought to the first voltage VpH tocause the switching transistor 105 a to be switched a plurality oftimes. Such an operation is repeated to cause the drive period and thestop period to be controlled such that the resistance-divided voltage Vnis brought to be a value between the first voltage VpH and the secondvoltage VpL to cause the output voltage V2 to be controlled to thepredetermined target voltage.

Here, when the resistance value of the third resistor 121 is set to R1,the resistance value of the fourth resistor 122 is set to R2, and theminimum switching frequency of the switching transistor 105 a is set toFmin, the capacitive element 123, which is connected in parallel withthe third resistor 121, is set to be a static capacitance C1 which meetsthe above-described Equation (1):

In the same manner as the third embodiment, this makes it possible tosuppress static load fluctuations of the output voltage to around thedifference between the first voltage and the second voltage when theload current i2 decreases to be a minimum switching frequency Fmin.

The ripple waveform of the output voltage V2 of the switching powersupply 101 b according to the fourth embodiment is described withreference to FIGS. 15A and 15B. The waveforms of the load current i2,the current iL of the choke coil 107, the output Vco of the comparator125 a, and the output voltage V2 are shown in FIG. 15A for a heavy loadwith a large load current i2 and in FIG. 15B for a light load with asmall load current i2. The capacitive element 123 may be connected todecrease the voltage ripple of the output voltage V2 from the product of(R1+R2)/R2 and the difference VpH−VpL of the first voltage and thesecond voltage to around the difference VpH−VpL of the first voltage andthe second voltage.

This is because the capacitive element 123 is connected to the thirdresistor 121 to stabilize the voltage Vr1 of the third resistor 121 toalmost a constant value. In this way, the voltage Vn of the invertinginput terminal of the comparator 125 a is brought to a value in which aVr1 is subtracted from the output voltage V2 to cause the voltage rippleof Vn to be equal to the voltage ripple of the output voltage V2. Thecontrol circuit 120 b controls the drive period and the stop period suchthat Vn is brought to be a value between VpL and VpH to control theoutput voltage V2 to a predetermined target voltage. Therefore, thevoltage ripple of Vn is brought to be equal to the difference VpH−VpLbetween the first voltage and the second voltage to also bring themagnitude of the voltage ripple of the output voltage v2 to VpH−VpL.

As V2=Vn+Vr1, the output voltage V2 is controlled to a value betweenVpH+Vr1 and VpL+Vr1.

When the output Vco of the comparator 125 a is brought to a first highlevel in t0, the switching transistor 105 a is turned on to increase thechoke coil current iL. When iL is brought to be larger than a value, theswitching transistor 105 a is turned off for a period, and, when Vcocontinually takes a first, high level, the switching transistor 105 a isturned on again. This causes switching to be performed a plurality oftimes during the period of t0-t1 in which the output Vco of thecomparator 125 a is at the first high level to bring the choke coilcurrent iL to almost a value. During the period in which Vco is at thefirst, high level, the choke coil current iL is larger than the loadcurrent i2, so that the output capacitor 108 a undergoes constantcurrent charging with the current of iL−i2 to cause the output voltageV2 to increase linearly. Moreover, when the Vco is at the second lowlevel, as shown for the period of t1-t2 of FIG. 15A, the choke coilcurrent iL is brought to 0. The output capacitor 108 a undergoesconstant current discharging only by the load current i2 to cause theoutput voltage V2 to decrease linearly.

Compared to the output voltage V2 according to the third embodiment inFIG. 11A, the output voltage V2 according to the fourth embodiment inFIG. 15A is seldom brought to be larger than VpH+Vr1 or smaller thanVpL+Vr1, so that it is suppressed to a value such that the magnitude ofthe voltage ripple of the output voltage V2 is suppressed to a valuewhich is closer to the hysteresis width of VpH−VpL. This is because,when the output Vco of the comparator 125 a is brought to the first highlevel at t0, iL reaches a current value of a constant current which islarger than i2 in a period which is sufficiently shorter than the lengthof the period t0-t1 in which Vco is at the first high level to cause theoutput capacitor 108 a to undergo constant current charging with iL−i2to cause V2 to increase linearly and, when the output Vco of thecomparator 125 a is brought to the second low level at t1, iL is broughtto 0 in a period which is sufficiently shorter than the length of theperiod t1-t2 in which Vco is at the second low level to cause the outputcapacitor 108 a to undergo constant current charging with i2 to cause V2to decrease linearly.

In FIG. 11A according to the third embodiment, as iL<i2 during the firsthalf t0-t1 and iL>i2 during the second half t1-t2 of the period in whichVco is at the first high level, charging and discharging of the outputcapacitor 108 a are performed, so that the voltage of 108 a at the timeof completion of the period in which Vco is at the first high level att2 is brought to be a center value of the voltage of 108 a, so that,with V2 being only the voltage of 108 a, V2 is brought to be larger thanVpH+Vr1, making it not possible to bring Vco to be at the second lowlevel, causing the hysteresis control not to perform correctly.Therefore, the output voltage V2 may be set to be the sum of the voltageof the output capacitor 108 a and the voltage of the equivalent seriesresistance 108 b of the output capacitor 108 a, with the magnitude whichis at least equivalent to the voltage ripple of 108 a, making itpossible to correctly operate the hysteresis control. Therefore, acapacitor with a large equivalent series resistance such as anelectrolytic capacitor is used for the output capacitor 108 a.

On the other hand, in a case of using switching to be performed aplurality of times in the period t0-t1 in which the output Vco of thecomparator 125 a is at the first high level, such as the fourthembodiment, with the charging/discharging of the output capacitor 108 a,the voltage of 108 a at t1 at the completion of the period in which Vcois at the first high level is brought to be the maximum value of thevoltage of 108 a, and the voltage of 108 a at t2 at the completion ofthe period in which Vco is at the second low level is brought to be theminimum value of the voltage of 108 a, making it possible to control V2to be between VpH+Vr1 and VpL+Vr1 without the voltage ripple of theequivalent series resistance 108 b, causing the hysteresis control tooperate correctly. In this way, a capacitor whose equivalent seriesresistance is small, such as a ceramic capacitor, may be used for theoutput capacitor 108 a. The ability to use a capacitor whose equivalentseries resistance is small is advantageous in reducing thehigh-frequency noise and dynamic load fluctuation of the output voltageV2 of the switching power supply 101 b.

With reference to FIG. 15B, a voltage ripple of the output voltage V2for the light load according to the fourth embodiment is described. Forthe light load, the period t4-t5 in which the output Vco of thecomparator 125 a is at the second low level increases to cause theswitching frequency to be low and the average value of iL to take thesame low value as i2. In the same manner as FIG. 11B according to thefourth embodiment. V2 has a short rise period and, in the fall periodthereof, causes the output capacitor 108 a to undergo constant currentdischarging with a low load current i2, causing V2 to decrease linearly.FIG. 15B has the same waveforms as those in FIG. 11B, so that, in thesame manner as what is described previously for the third embodiment,the average value of the inverting output terminal voltage Vn of thecomparator 125 a that is shown in FIG. 12A decreases from the centervalue to the same extent as the voltage ripple ΔVr1 of theinter-terminal voltage Vr1 of the third resistor 121. This results inthe static load fluctuation ΔV2L of the product of (R1+R2)/R2 and ΔVr1,so that it is necessary to connect the capacitive element 123 whichmeets Equation (1) in parallel with the third resistor to suppress thisstatic load fluctuation ΔV2L to around the voltage ripple ΔV2 of theoutput voltage V2.

As described above, for the control circuit of the present embodiment,it is desirable to cause the switching transistor 105 a to be switched aplurality of times in a period in which the output Vco of the comparator125 a is at the first level (high level). In this way, as the on/offperiod of the switching transistor 105 a is sufficiently shorter thanthe on/off period of the output Vco of the comparator 125 a, the outputVco of the comparator 125 a being brought to the first level (highlevel) causes the output voltage V2 to increase, while the output Vco ofthe comparator 125 a being brought to the second level (low level)causes the output voltage V2 to decrease, making it possible to suppressthe output voltage ripple to around the difference between the firstvoltage and the second voltage.

Moreover, for the control circuit according to the present embodiment,it is desirable to turn off the switching transistor 105 a for a periodwhen the current which flows through the switching power supply 101 b isbrought to at least a value. This causes the current which flows throughthe switching power supply 101 b to be brought to at least a value inthe period in which the output Vco of the comparator 125 a is at thefirst level (high level) to cause repeating of the switching transistor105 a to be turned off for a period, making it possible to causeperforming of switching a plurality of times during a period in whichthe output Vco of the comparator 125 a is at the first level. This makesit possible to cause the output capacitor 108 a of the switching powersupply 101 b to be charged with a current which is larger than the loadcurrent in the period in which the output Vco of the comparator 125 a isat the first level, so that the output voltage V2 linearly increasessimultaneously with the beginning of the period in which the outputvoltage V2 is at the first level. The output capacitor 108 a of theswitching power supply 101 b is discharged with only the load current inthe period in which the output Vco of the comparator 125 a is at thesecond level (low level), so that the output voltage V2 linearlydecreases at the same time as the beginning of the second level. Thismakes it possible to suppress the output voltage ripple to around thedifference between the first voltage and the second voltage.

For the current which flows through the switching power supply, variousdetection methods are possible depending on the configuration of theswitching power supply. For example, the switching power supply 101 bshown in the present embodiment may detect the current of the switchingtransistor 105 a or the choke coil 107. Moreover, an insulation-typeswitching power supply may detect the coil current of the transformer,while the current of a resonant capacitor may be detect when theresonant capacitor is present.

Fifth Embodiment

FIG. 16 is a circuit diagram illustrating a configuration of a switchingpower supply 101 c according to a fifth embodiment of the presentinvention. As one example, the switching power supply 101 c shown inFIG. 16 includes a pair of input terminals 102 a and 102 b (also called“an input terminal 102” when not specifically distinguishingtherebetween); a pair of output terminals 103 a and 103 b (also called“an output terminal 103” when not specifically distinguishing between);a main circuit 104 c; and a control circuit 120 c, and converts an inputvoltage (DC voltage) V1 input to the input terminal 2 to an outputvoltage (DC voltage) V2 to output the converted result from the outputterminal 103 and controls the output voltage V2 to be at a predeterminedtarget voltage. The switching power supply 101 c inputs an input voltageV1 and an input current i1 to the input terminal 102 to output theoutput voltage V2 and a load current i2 from the output terminal 103.

The main circuit 104 c includes a switching transistor 105 a, a parasitediode 105 b of the switching transistor 105 a, a diode 106, resonantinductors 109, 113, 114, and 117, resonant capacitors 110, 111, 112,115, and 116, an output capacitor 108 a, and an equivalent seriesresistor 108 b of the output capacitor 108 a. The switching power supply101 c, which is configured with a resonant converter circuit scheme asone example of the switching power supply, outputs an input voltage V1input from the input terminal 102 to the output voltage V2 to output theconverted result to the output terminal 103.

The control circuit 120 c includes a third resistor 121 and a fourthresistor 122 that voltage-divides the output voltage V2 of the switchingpower supply 101 c; a comparator 125 a in which a voltage-dividedvoltage Vn is input to a first inverting input terminal thereof and areference voltage Vp is input to a second non-inverting input terminalthereof, and a controller 130 y which controls a switching transistor105 a based on an output signal Vco of the comparator 125 a, wherein thereference voltage Vp is brought to be a first high VpH when an outputVco of the comparator 125 a takes a first high level and is brought tobe a second low VpL when the output Vco of the comparator 125 a takes asecond low level. The control circuit 120 c includes the third resistor121, which is connected between a positive terminal 103 a of an outputterminal and a first inverting input terminal of the comparator 125 a;and a capacitive element 123 which is connected in parallel with thethird resistor 121. A common ground G of the control circuit 120 c isconnected to a negative terminal 103 b of the output terminal. A voltageof each of signal voltages Vn, Vp, and Vco is set to be a voltage withthe common ground G as the reference.

Moreover, a so-called hysteresis comparator is provided in the samemanner as the third embodiment in which a reference voltage Vp isbrought to be a first high voltage VpH when an output Vco of thecomparator 125 a takes a first high level and is brought to be a secondlow voltage VpL when an output Vco of the comparator 125 a takes asecond low level. As an example of a hysteresis comparator circuitscheme, in the same manner as the third embodiment, the switching powersupply 101 c includes the comparator 125 a; a resistor 125 b whichconnects between a non-inverting input terminal and an output terminalof the comparator 125 a; and a resistor 125 c and a constant voltagesource 124 that connects in series between the non-inverting inputterminal and the common ground G.

The controller 130 y is configured with an oscillator 130 f whichoscillates at a frequency, and a drive device 130 e which drives theswitching transistor 105 a based on the output Vco of the comparator 125a and the output of the oscillator 130 f.

When the output Vco of the comparator 125 a is brought to the first highlevel, the drive device 130 e turns on and off the switching transistor105 a based on an output of the oscillator 130 f which oscillates at thefrequency. The inter-terminal voltage V5 of the switching transistor 105a increases from 0V by turning off the switching transistor 105 a. Theresonant capacitor 111 may be charged to cause the inter-terminalvoltage V5 to start increasing only after the switching transistor 105 ais turned off completely to decrease the switching loss at the time ofturning off the switching transistor 5 a. Thereafter, the inter-terminalvoltage V5 again returns to 0V when the switching transistor 105 a isturned off for a period by resonating of the resonant inductors 109,113, 114, 17, and the resonant capacitors 110, 111, 112, 115, and 116.Then, the switching transistor 105 a may be turned on to result inso-called ZVS (zero voltage switching), making it possible to decrease aswitching loss at the time of turning on the switching transistor 105 a.When the output Vco of the comparator 125 a continues at the first highlevel, a process is repeated such that the switching transistor 105 a isturned off after being turned during a period, making it possible tocausing switching to be performed a plurality of times in the period inwhich the output Vco of the comparator 125 a is at the first high level.

Next, an operation of the control circuit 120 c is described. When thevoltage in which the output voltage V2 is resistance-divided is broughtto be lower than the second voltage VpL, the output Vco of thecomparator 125 a is brought to the first high level, and the switchingdevice 130 y starts driving of the switching transistor 105 a, and thereference voltage Vp is brought to the first voltage VpH. The switchingtransistor 105 a being turned on and off by the controller 130 y causesa current iD having a semi-sinusoidal wave with a peak value to flowfrom the diode 106. The time average of iD being larger than the loadcurrent i2 causes the output capacitor 108 a to be charged to cause theoutput voltage V2 to increase. In the controller 130 y, when the voltageVn in which the output voltage V2 is resistance-divided is brought to behigher than the first voltage VpH, the comparator output Vco is broughtto be at the second low level to cause the drive period to be completedto be transitioned to a stop period. Then, the reference voltage Vp isbrought to the second voltage VpL. In the stop period, the current iDfrom the diode 106 is brought to 0, causing discharging from the outputcapacitor 108 a by the load current i2 to cause the output voltage V2 todecrease. When the voltage Vn in which the output voltage V2 isresistance-divided is brought to be lower than the second voltage VpLagain, the drive period is started again to bring the reference voltageVp to the first voltage VpH to switch the switching transistor 105 a aplurality of times. Repeating this operation causes the drive period andthe stop period to be controlled such that the resistance-dividedvoltage Vn is brought to a value between the first voltage VpH and thesecond voltage VpL to cause the output voltage V2 to be controlled to apredetermined target voltage.

Here, the capacitive element 123 which is connected in parallel with thefirst resistor 121 is set to be a static capacitance C1 which meets theabove-shown Equation (1) when the resistance value of the third resistor121 is set to R1, the resistance value of the fourth resistor 122 is setto R2, and the minimum switching frequency of the switching transistor105 a is set to Fmin.

In this way, in the same manner as the third and fourth embodiments,when the load current i2 decreases to result in the minimum switchingfrequency Fmin, the static load fluctuation of the output voltage may besuppressed to around the difference between the first voltage and thesecond voltage.

With reference to FIGS. 17A and 17B, a ripple waveform of the outputvoltage V2 of the switching power supply 101 c according to the fifthembodiment is described. The waveform of the load current i2 for a heavyload when the load current i2 is large is shown along in FIG. 17A andthe waveform of the load current i2 for a light load when the loadcurrent i2 is small is shown in FIG. 17B, along with the waveforms ofthe current iD of the diode 106, the output Vco of the comparator 125 a,and the output voltage V2. The capacitive element 123 may be connectedto decrease the voltage ripple of the output voltage V2 from the productof (R1+R2)/R2 and the difference VpH−VpL of the first voltage and thesecond voltage to around the difference VpH−VpL of the first voltage andthe second voltage.

This is because the capacitive element 123 is connected to the thirdresistor 121 to stabilize the voltage Vr1 of the third resistor 121 toalmost a constant value. In this way, the voltage Vn of the invertinginput terminal of the comparator 125 a is brought to a value in which aVr1 is subtracted from the output voltage V2 to cause the voltage rippleof Vn to be equal to the voltage ripple of the output voltage V2.

The control circuit 120 c controls the drive period and the stop periodsuch that Vn is brought to be a value between VpL and VpH to control theoutput voltage V2 to a predetermined target voltage. Therefore, thevoltage ripple of Vn is brought to be equal to the difference VpH−VpLbetween the first voltage and the second voltage to also bring themagnitude of the voltage ripple of the output voltage v2 to VpH−VpL. AsV2=Vn+Vr1, the output voltage V2 is controlled to a value betweenVpH+Vr1 and VpL+Vr1.

When the output Vco of the comparator 125 a is brought to a first highlevel in t0, the switching transistor 105 a is turned on and off by thecontroller 130 y to cause the current iD having a semi-sinusoidal wavewith a peak value to flow through the diode 106. The time average of iDis constant and larger than the load current i2, causing the outputcapacitor 8 a to undergo constant-current charging in the period oft0-t1 to cause the output voltage V2 to increase linearly. Moreover,during the period of t1-t2 in which the Vco is at the second low level,the current iD of the diode 106 is brought to 0 as shown in FIG. 17A.The output capacitor 108 a undergoes constant current discharging onlyby the load current i2 to cause the output voltage V2 to decreaselinearly.

In the same manner as the output voltage V2 according to the fifthembodiment in FIG. 17A, the output voltage V2 according to the fourthembodiment in FIG. 15A is seldom brought to be larger than VpH+Vr1 orsmaller than VpL+Vr1, so that it is suppressed to a value such that themagnitude of the voltage ripple of the output voltage V2 is suppressedto a value which is closer to the hysteresis width of VpH−VpL. This isbecause, when the output Vco of the comparator 125 a is brought to thefirst high level at t0, iD reaches a current value of a constant currentwhich is larger than i2 in a period which is sufficiently shorter thanthe length of the period t0-t1 in which Vco is at the first high levelto cause the output capacitor 108 a to undergo constant current chargingwith iL−i2 to cause V2 to increase linearly and, when the output Vco ofthe comparator 125 a is brought to the second low level at t1, iD isbrought to 0 in a period which is sufficiently shorter than the lengthof the period t1-t2 in which Vco is at the second low level to cause theoutput capacitor 108 a to undergo constant current discharging with i2to cause V2 to decrease linearly.

Moreover, in the same manner as the fourth embodiment, according to thefifth embodiment, for causing switching to be performed a plurality oftimes in the period in which the output Vco of the comparator 125 a isat the first high level, charging and discharging of the outputcapacitor 108 a cause the voltage of the output capacitor 108 a at thetime of completion of the period in which Vco is at the first high levelat t1 is brought to be the maximum value of the voltage of the outputcapacitor 108 a. Moreover, the voltage of the output capacitor 108 a atthe time of completion of the period in which the output Vco of thecomparator 125 a is at the second low level at t2 is brought to be theminimum value of the voltage of the output capacitor 108 a. Thus, evenwhen the voltage ripple of the equivalent series resistance 108 b is notpresent, the output voltage V2 may be controlled to be between VpH+Vr1and VpL+Vr1, causing the hysteresis control to operate correctly.Therefore, a capacitor with a small equivalent series resistance, suchas a ceramic capacitor, may be used for the output capacitor 108 a. Thecapacitor with the small equivalent series resistance may be used toachieve advantageous effects in reducing the dynamic load fluctuation ofthe output voltage V2 of the switching power supply 101 c and the highfrequency noise. Moreover, according to the fifth embodiment, theswitching frequency may be increased while suppressing the switchingloss of the switching transistor 105 a to decrease energy whichaccumulates in the resonant inductors 109, 113, 114, 117 and theresonant capacitors 110, 111, 112, 115, 116 that are used in the maincircuit 104 c of the resonant converter. The smaller the energyaccumulated in these resonant inductors and capacitors, the greater theability to cause the average value of the current iD of the diode 106 tofollow a value which is close to the load current i2 when the loadcurrent i2 changes rapidly, making it possible to suppress the dynamicload fluctuation of the output voltage. This makes it possible tosuppress the dynamic load fluctuation of the output voltage, the staticload fluctuation, and the output voltage ripple to around the differencebetween the first voltage and the second voltage.

With reference to FIG. 17B, a voltage ripple of the output voltage V2for the light load according to the fifth embodiment is described. Forthe light load, the period t4-t5 in which the output Vco of thecomparator 125 a is at the second low level increases to cause theswitching frequency to be low. In the same manner as FIG. 11B accordingto the third embodiment and FIG. 15B according to the fourth embodiment,V2 has a short rise period and, in the fall period thereof, causes theoutput capacitor 108 a to undergo constant current discharging over along time with a low load current i2, causing V2 to decrease linearly.FIG. 17B has the same waveforms as those in FIGS. 11B and 15B, so that,in the same manner as what is described previously for the first andsecond embodiments, the average value of the inverting output terminalvoltage Vn of the comparator 125 a that is shown in FIG. 12A decreasesfrom the intermediate value to the same extent as the voltage rippleΔVr1 of the inter-terminal voltage Vr1 of the third resistor 121. Thisresults in the static load fluctuation ΔV2L of the product of (R1+R2)/R2and ΔVr1, so that it is necessary to connect the capacitive element 123which meets Equation (1) in parallel with the third resistor 121 tosuppress this static load fluctuation ΔV2L to around the voltage rippleΔV2 of the output voltage V2.

As described above, the control circuit 120 c of the present embodimentis a resonant converter in which the main circuit 104 c of the switchingpower supply 101 c includes a resonant converter. In this way, theswitching frequency may be increased while suppressing the switchingloss to decrease energy accumulated in the inductors 109, 113, 114, 117and capacitors 110, 111, 112, 115, 116. Therefore, the dynamic loadfluctuation of the output voltage V2, the static load fluctuation, andthe output voltage ripple may be suppressed to around the differencebetween the first voltage and the second voltage.

Sixth Embodiment

A sixth embodiment according to the present invention is described forthe configuration of a switching power supply device 101 d withreference to the drawings. The switching power supply device 101 d shownin FIG. 18 is an addition of a charging circuit 140 which charges thecapacitive element 123 at the time of activation to the configuration ofthe fifth embodiment shown in FIG. 16. The same charging circuit 140 mayalso be added to the third and fourth embodiments. Describing the fifthembodiment as an example, the capacitive element 123 connects thecapacitive element which meets Equation (1) in parallel with the thirdresistor 121, so that a period at the time of activating the switchingpower supply 101 c shown in FIG. 16 in which the inter-terminal voltageVr1 of the third resistor 121 is lower than the voltage Vr1, which isstable after completion of the activation, continues for a long time.The inverting input terminal voltage Vn of the comparator 125 a iscontrolled to V2=Vp+Vr1 since the output voltage V2 is controlled to bebrought to the same value as the reference voltage Vp, so that a periodcontinues for a long time in which the output voltage V2 also is lowerthan the predetermined target voltage. The predetermined target voltagefor the output voltage V2 is the product of (R1+R2)/R2 and Vp, so thatVr1 then is Vp times R1/R2. For Vr1 to reach the voltage of Vp×R1/R2from 0V at the time of activation, the capacitive element 123 is chargedalong a route which goes through the capacitive element 123, the fourthresistor 122, and the common ground G from the positive terminal 103 aof the output terminal. Taking into account the loss of the controlcircuit, resistance values R1 and R2 of the third resistor 121 and thefourth resistor 122 may not be set to a small value (for example, lessthan or equal to 1 kW). Therefore, the charge current of the capacitiveelement 123 being small and the static capacitance of the capacitiveelement 123 shown in Equation (1) being large cause the period to reachthe voltage Vp×R1/R2 from 0V at the time of activation to be longer.

The charging circuit 140 shown in FIG. 18 detects the output voltage V2to short-circuit the common ground G and the inverting input terminal ofthe comparator 125 a only in an activation period in which the outputvoltage V2 is also lower than the predetermined target voltage.Therefore, during the activation period, the capacitive element 123 ischarged with a large charge current along a route passes through thecapacitive element 123 and the charging circuit 140 from the positiveterminal 103 a of the output terminal. In this way, the activationperiod for the output voltage V2 to reach the predetermined targetvoltage from 0V may be shortened when the capacitive element 123 of thestatic capacitance shown in Equation (1) is connected in parallel withthe third resistor 121.

FIGS. 19A and 19B show embodiments of a so-called hysteresis comparatorin which a reference voltage Vp is brought to a first high voltage VpHwhen an output Vco of the comparator takes a high level VcoH and to asecond voltage VpL when the output Vco of the comparator takes a secondlow voltage VcoL. For the hysteresis comparator circuit scheme, a firstspecific example is shown in FIG. 19A, while a second specific exampleis shown in FIG. 19B. The first specific example of the hysteresiscomparator circuit scheme includes a comparator 125 a; a resistor 125 bof a resistance value Rb that connects between the non-inverting inputterminal and the output terminal of the comparator 125 a; and a constantvoltage source 124 of a voltage V24 and a resistor 125 c of a resistancevalue Rc that connects serially between the common ground G and thenon-inverting terminal. The reference voltage Vp when the output Vco ofthe comparator 125 a is at the first high level VcoH is brought to VpH,shown in Equation (20) below:

$\begin{matrix}{V_{pH} = {{V_{24}\frac{R_{b}}{R_{b} + R_{c}}} + {V_{coH}\frac{R_{c}}{R_{b} + R_{c}}}}} & (20)\end{matrix}$

The reference voltage Vp when the output Vco of the comparator is at thesecond low level VcoL is brought to VpL, shown in Equation (21) below:

$\begin{matrix}{V_{pL} = {{V_{24}\frac{R_{b}}{R_{b} + R_{c}}} + {V_{coL}\frac{R_{c}}{R_{b} + R_{c}}}}} & (21)\end{matrix}$

The difference VpH−VpL between the first voltage and the second voltageis shown in Equation (22) below:

$\begin{matrix}{{V_{pH} - V_{pL}} = {\left( {V_{coH} - V_{coL}} \right)\frac{R_{c}}{R_{b} + R_{c}}}} & (22)\end{matrix}$

As an example, when VcoH=5V in which the output Vco of the comparator125 a is equal to the power supply voltage of the comparator 125 a, orVcoL=0V, and V24=1.5V, and Rc/(Rb+Rc)=0.02 in which the resistance valueRc of the resistor 125 c is set to be sufficiently smaller than theresistance value Rb of the resistor 125 b, VpH=1.57V, VpL=1.47V, andVpH−VpL=0.1V.

The inverting input terminal voltage Vn of the comparator 125 a iscompared with a high VpH to determine whether Vco is changed from VcoHto VcoL in the period in which the output Vco of the comparator 125 a isat the first high level VcoH and the inverting input terminal voltage Vnof the comparator 125 a is compared with a low VpL to determine whetherVco is changed from VcoL to VcoH in the period in which the output Vcoof the comparator 125 a is at the second low level VcoL, the output Vcoof the comparator 125 a is not changed and maintained to the first highlevel VcoH or the second low level VcoL when Vn takes an intermediatevalue of VpH and VpL.

A second example shown in FIG. 19B is a so-called window comparatorwhich may realize a function equivalent to that of the hysteresiscomparator according to the first specific example. The second specificexample includes a comparator 125 g, a comparator 125 h, a constantvoltage source 124 b of a voltage VpH, a resistor 125 j whichvoltage-divides the constant voltage source 124 b, a resistor 125 k, andan SR flip-flop 125 f. The input signal Vn to the window comparator isinput to the non-inverting input terminal of the comparator 125 g andthe inverting input terminal of the comparator 125 h; the voltage VpH ofthe constant voltage source 124 b is input to the inverting inputterminal of the comparator 125 g, and the voltage VpL in which thevoltage VpH of the constant voltage source 124 b is voltage-divided bythe resistor 125 j and the resistor 125 k is input to the non-invertinginput terminal of the comparator 125 h. The output of the comparator 125h is brought to a high level to set the SR flip-flop 125 f, and theoutput Vco of the SR flip-flop 125 f to be brought to the first highlevel VcoH, and the output of the comparator 125 g is brought to a highlevel to reset the SR flip-flop 125 f, and the output Vco of the SRflip-flop 125 f to be brought to the second low level VcoL. In this way,in the period in which the output Vco of the SR flip-flop 125 f is atthe first high level VcoH, the input signal Vn to the window comparatoris compared with the high VpH to determine whether Vco is changed fromVcoH to VcoL and, in the period in which the output Vco of the SRflip-flop 125 f is at the second low level VcoL, the input signal Vninto the window comparator is compared with the low VpL to determinewhether the Vco is changed from VcoL to VcoH, Vco is not changed andmaintained at the first high level VcoH or the second low level VcoLwhen Vn takes an intermediate value of VpH and VpL.

Therefore, the window comparator shown in FIG. 19B may realize thefunction of the input signal Vn and the output signal Vco that areequivalent to the hysteresis comparator shown in FIG. 19A.

The function equivalent to the hysteresis comparator and the windowcomparator according to the embodiments of the present invention thatare described above may be embedded into an integrated circuit, so thatthe embodiments of the present invention are not to be limited to theabove-recited descriptions of the embodiments.

Seventh Embodiment

FIG. 20 is a circuit diagram illustrating the configuration of aswitching power supply 201 a according to a seventh embodiment of thepresent invention. As one example, the switching power supply 201 ashown in FIG. 20 includes a pair of input terminals 202 a and 202 b(also called “an input terminal 202” when not specificallydistinguishing therebetween); a pair of output terminals 203 a and 203 b(also called “an output terminal 203” when not specificallydistinguishing between); a main circuit 204 a; and a control circuit 220a, and converts an input voltage (DC voltage) V1 input to the inputterminal 202 to an output voltage (DC voltage) V2 to output theconverted result from the output terminal 203 and controls the outputvoltage V2 to be at a predetermined target voltage. The switching powersupply 201 a inputs an input voltage V1 and an input current i1 to theinput terminal 202 to output the output voltage V2 and a load current i2from the output terminal 203. The switching power supply 201 a inputs aninput voltage V1 and an input current i1 to the input terminal 202 tooutput the output voltage V2 and a load current i2 from the outputterminal 203.

The main circuit 204 a includes a switching transistor 205 a, a parasitediode 205 b of the switching transistor 205 a, a diode 206, a choke coil207, an output capacitor 208 a, and an equivalent series resistor 208 bof the output capacitor 208 a. The switching power supply 201 a, whichis configured with a back converter circuit scheme as one example,outputs an input voltage V1 input from the input terminal 202 to theoutput voltage V2 to output the converted result to the output terminal3.

The control circuit 220 a includes a third resistor 221 and a fourthresistor 222 that voltage-divide the output voltage V2 of the switchingpower supply 201 a; a comparator 225 a in which a voltage-dividedvoltage Vn is input to a first inverting input terminal thereof and areference voltage Vp is input to a second non-inverting input terminalthereof; and a controller 230 which controls a switching transistor 205a based on an output signal Vco of the comparator 225 a. The controlcircuit 220 a includes the third resistor 221, which is connectedbetween a positive terminal 203 a of an output terminal and a firstinverting input terminal of the comparator 225 a; and a seriesconnection of a capacitive element 223 and a switch element 226 that isconnected in parallel with the third resistor 221. A reference voltageVp is brought to be a first high voltage VpH when an output Vco of thecomparator 225 a takes a first high level and to be a second low voltageVpL when an output Vco of the comparator 225 a takes a second low level.Moreover, the control circuit 220 a includes an OFF period detectioncircuit 250 which detects that the second level of the output Vco of thecomparator 225 a has been continuing longer than a period, wherein, whenthe second level of the output Vco of the comparator 225 a has beencontinuing longer than the period, the OFF period detection circuit 250opens the switch element 226 with an output signal Vs and, at the sametime, brings the reference voltage Vp to a third intermediate voltageVpM with an output signal Vm.

A common ground G of the control circuit 220 a is connected to anegative terminal 203 b of the output terminal. Each of signal voltagesVn, Vp, Vco, Vs, and Vm is set to be a voltage with the common ground Gas the reference.

Moreover, a so-called hysteresis comparator is known in which thereference voltage Vp is brought to be a first high voltage VpH when theoutput Vco of the comparator 225 a takes a first high level and to be asecond low voltage VpL when the output Vco of the comparator 225 a takesa second low level. Furthermore, when the second low level of the outputVco of the comparator 225 a has been continuing longer than a specificperiod, the reference voltage Vp is brought to be a third intermediatevoltage VpM with an output signal Vm of the OFF period detection circuit50. The switching power supply 201 is configured with a hysteresiscomparator circuit scheme as an example and includes the comparator 225a; a resistor 225 b which connects between a non-inverting inputterminal and an output terminal of the comparator 225 a; and a resistor225 c and a constant voltage source 224 that are connected in seriesbetween the common ground G and the non-inverting input terminal; and aresistor 225 d which is connected between the non-inverting inputterminal of the comparator 225 a and the OFF period detection circuit250.

Next, an operation of the control circuit 220 a is described. When thevoltage Vn in which the output voltage V2 is resistance-divideddecreases below the second voltage VpL, the comparator output Vco isbrought to a first high level to cause the controller 230 to startdriving the switching transistor 205 a, so that the reference voltage isbrought to be the first voltage VpH. During the drive period of theswitching transistor 205 a, the current of the choke coil 207 increasesto cause a current iL which is larger than a load current i2 to besupplied from the choke coil 207 to the output capacitor 208 a, causingthe output capacitor 208 a to be charged to raise the output voltage V2.When the voltage Vn in which the output voltage V2 is resistance-dividedis brought to be higher than the first voltage VpH, the comparatoroutput Vco is brought to a second low level to cause the drive period tobe completed and to be transitioned to a stop period. At this time, thereference voltage is brought to be the second voltage VpL. In the stopperiod, the load current i2 is brought to be larger than the current iLfrom the choke coil 207 to cause discharging from the output capacitor208 a to cause the output voltage V2 to decrease. When the voltage Vn inwhich the output voltage V2 is resistance-divided is again brought to belower than the second voltage VpL, the drive period is started again tobring the reference voltage to the first voltage VpH. Such an operationis repeated to cause the drive period and the stop period to becontrolled such that the resistance-divided voltage Vn is brought to bea value between the first voltage VpH and the second voltage VpL tocause the output voltage V2 to be controlled to the predetermined targetvoltage.

Here, when the switching frequency is high and the period in which theoutput Vco of the comparator 225 a is at the second low level is short,the switch element 226 is closed. In this way, the capacitive element223 is connected to the third resistor 221 to cause an output voltageripple to be suppressed. When the switching frequency decreases and theperiod in which the output Vco of the comparator 225 a is at the secondlow level increases, the switch element 226 is opened to cause thecapacitive element 223 to be separated from the third resistor 221 andthe reference voltage to be a third voltage VpM.

Next, an effect of suppressing the output voltage fluctuations by thecapacitive element 223, which is connected to the third resistor 221, isdescribed using a waveform of the output voltage V2. FIG. 28 is acircuit diagram illustrating the configuration of the switching powersupply 201 e according to the related art. Compared to the circuitaccording to the seventh embodiment of the present invention, neitherthe switch element 226 nor the OFF period detection circuit 250 areincluded in FIG. 28. In FIG. 28, the capacitive element 223 is connectedin parallel with the third resistor 221, while the OFF period detectioncircuit 250 is not connected, so that, a case in which the switchingfrequency is high and the OFF period detection circuit 250 is notoperating in FIG. 20 is equivalent to FIG. 28. FIG. 21 is a timingwaveform diagram illustrating a relationship between the capacitiveelement 223 and the output voltage. The waveform of the output voltageV2, which is schematically shown in FIG. 21, is a waveform which isextended in the Y-axis direction except for the DC component of theoutput voltage V2. The X axis shows time, while the Y axis shows avoltage ripple of the output voltage. Moreover, a voltage ripple of theoutput voltage V2 and static load fluctuations are shown in FIG. 21, forcases of (a) in which the capacitive element 223 is further removed fromFIG. 28; (b) in FIG. 28 in which only the capacitive element 223 isconnected; and (c) in FIG. 20 in which the circuit according to thepresent embodiment is connected. Moreover, in FIG. 21, each of (a), (b),and (c) shows cases in which the load is a rated load and in which theload is a light load.

In a case without the capacitive element 223 that is shown in (a) inFIG. 21, the voltage ripple of the output voltage V2 is increased up tothe product of a voltage-dividing ratio (R1+R2)/R2 and the differenceVpH−VpL in voltage between the first voltage and the second voltage.There are no changes in the voltage ripple regardless of whether theload connected to the output terminal 203 is a rated load or a lightload, and, for the light load, the switching frequency is lower and thestop period is longer.

In a case shown in (b) in FIG. 21 in which only the capacitive element223 is connected, the voltage ripple of the output voltage V2 may bedecreased to around the difference VpH−VpL between the first voltage andthe second voltage. Regardless of whether the load connected to theoutput terminal 203 is the rated load or the light load, the voltageripple may be decreased to around the same level. However, the averagevalue of the output voltage V2 differs between the rated load and thelight load, so that there are large static load fluctuations.

In a case shown in (c) in FIG. 21 in which the circuit according to thepresent embodiment is connected, the static load fluctuations, whichrepresent a difference between the light load and the rated load of theaverage value of the output voltage V2, in addition to the voltageripple of the output voltage V2 also may be suppressed. Therefore, theoutput voltage fluctuations may be suppressed.

Next, the reason that the voltage ripple of the output voltage V2 isbrought to the product of the voltage-dividing ratio (R1+R2)/R2 and thedifference VpH−VpL between the first voltage and the second voltage whenthe capacitive element 223 is not connected is described. The outputvoltage V2 is voltage-divided between the third resistor 221 and thefourth resistor 222, so that the voltage Vn, which is input to theinverting input terminal of the comparator 225 a, is voltage-divided tothe product of R2/(R1+R2) and the output voltage V2. In this way, thevoltage ripple of Vn is also attenuated to the product of R2/(R1+R2) andthe voltage ripple of the output voltage V2. The control circuit 220 acontrols the drive period and the stop period such that Vn is brought toa value between VpL and VpH to control the output voltage V2 to apredetermined target voltage. Therefore, the voltage ripple of Vn isbrought to be equal to the difference VpH−VpL of the first voltage andthe second voltage to cause the voltage ripple of the output voltage V2to increase up to the product of (R1+R2)/R2 and the voltage ripple ofVn, so that the magnitude of the voltage ripple of the output voltage V2is brought to be (VpH−VpL)(R1+R2)/R2.

Next, the reason is described that the capacitive element 223 may beconnected to decrease the voltage ripple of the output voltage V2 fromthe product of (R1+R2)/R2 and the difference VpH−VpL of the firstvoltage and the second voltage to around the difference VpH−VpL of thefirst voltage and the second voltage. The capacitive element 223 isconnected to the third resistor 221 to cause the voltage Vr1 of thethird resistor 221 to be stabilized to almost a value. In this way, thevoltage Vn of the inverting input terminal of the comparator 225 a isbrought to a value in which a Vr1 is subtracted from the output voltageV2 to cause the voltage ripple of Vn to be equal to the voltage rippleof the output voltage V2. The control circuit controls the drive periodand the stop period such that Vn is brought to be a value between VpLand VpH to control the output voltage V2 to a predetermined targetvoltage. Therefore, the voltage ripple of Vn is brought to be equal tothe difference VpH−VpL between the first voltage and the second voltageto also bring the magnitude of the voltage ripple of the output voltageV2 to VpH−VpL.

As V2=Vn+Vr1, the output voltage V2 is controlled to a value betweenVpH+Vr1 and VpL+Vr1.

However, the voltage ripple of the output voltage V2 is brought to alittle larger than (VpH−VpL)(R1+R2)/R2 in the case without thecapacitive element 223 as shown in (a) in FIG. 21, and is brought to bea little larger than VpH−VpL in the case of connecting the capacitiveelement 223 as shown in (b) and (c) in FIG. 21. The reason is due to thefact that the output voltage V2 reaches the maximum, before it starts todrop, later than the completion of the drive period of the switchingtransistor 205 a and the output voltage V2 reaches the minimum, beforeit starts to rise, later than the completion of the stop period of theswitching transistor 205 a. This is similar to the ripple waveform ofthe output voltage V2 of FIGS. 10A and 10B. In other words, theoperating waveform when the coil current iL of the switching powersupply 201 a is operating in the current continuous mode in which thecoil current iL of the switching power supply 201 a is continuouslychanged in the same as FIG. 10A, and, moreover, the operating waveformwhen the coil current iL of the switching power supply 201 a isoperating in the current non-continuous mode in which the coil currentiL of the switching power supply 201 a is non-continuously changed inthe same as FIG. 10B, so that the similar explanations for theabove-mentioned figures will be omitted.

The timing waveform diagram for explaining the operation when thecapacitive element 223 of the switching power supply in FIG. 20 isconnected is the same as FIGS. 11A and 11B, so that the similarexplanations for the above-mentioned figures will be omitted.

Next, using FIGS. 12A and 12B, the reason for the fact that the averagevalue of the output voltage V2 is lower than a case of the rated load ina case of a light load is described in a case in which the capacitanceelement 223 of the switching power supply is connected in (b) in FIG.21.

FIG. 12A is a timing waveform diagram for a case of connecting only thecapacitive element 223 in which the switching frequency decreases in thelight load, while FIG. 12B is a timing waveform diagram for a case inwhich the load is heavy and the switching frequency does not decrease.In FIGS. 12A and 12B. Vco is an output of the comparator 225 a; V2 is anoutput voltage; Vr1 is a voltage of the voltage-dividing resistor 221;Vn is an inverting input terminal voltage of the comparator 225 a;Vn_AVG is the average value of Vn; ir1 is a current of thevoltage-dividing resistor 221; and ir2 is a current of thevoltage-dividing resistor 222.

FIG. 12A shows the state of the control circuit 220 a when operated withthe light load and the current non-continuous mode. The comparator 225 ahas a short period of a high output level and a long period of a lowoutput level, causing the waveform of the output voltage V2 to have ashort period in which the voltage increases and to have a long period inwhich the voltage decreases with a triangular wave in which the voltagelinearly decreases.

FIG. 22 shows the relationship between the frequency and the gain of alow-pass filter which has V2 as an input and Vr1 as an output withvoltage-dividing resistors 221 and 222, and the capacitive element 223.The transmission function of the low-pass filter is to take theabove-shown Equation (2).

Explanations for Equations (2) to (7) according to the third embodimentis omitted since they are similar in FIG. 22. The switching frequencynot decreasing because of the heavy load causes the waveform in FIG. 12Bto be generated. Moreover, explanations for Equations (9)-(10) accordingto the third embodiment that are similar in FIG. 22 will be omitted.

The difference ΔV2L of the average value of V2 in FIG. 12A and theaverage value of V2 in FIG. 12B is a static load fluctuation for a caseof connecting only the capacitive element 223, which is the differencebetween the average value of V2 for the case in which the switchingfrequency decreases for the light load in FIG. 12A and the average valueof V2 for the case in which the switching frequency does not decreasefor the heavy load in FIG. 12B from Equations (3), (7), and (10), thedifference ΔV2L of the average value of V2 is to take the above-shownEquation (11).

Explanations for Equation (12) according to the third embodiment will beomitted as they are the same in FIG. 22.

Next, using FIG. 23, the fact is explained that the circuit according tothe present embodiment may be connected to suppress the static loadfluctuation ΔV2L of the output voltage as shown in (c) in FIG. 21.

When the output Vco of the comparator 225 a is brought to a first highlevel in the period from t0 to t1, the output voltage V2 increases tocause the inverting input terminal voltage Vn of the comparator 225 a toalso increase. When the Vn reaches the first reference voltage VpH, theVco is brought to a second low level, while, when the switchingfrequency decreases in the non-continuous mode, a current iL which issupplied from the choke coil 207 to the output capacitor 208 a isbrought to 0, and the load current i2 causes the output voltage V2 todecrease linearly. When the switching frequency decreases with the lightload, a period from t1 in which the comparator output Vco takes a lowlevel continues for a long time.

The OFF period detection circuit 250 detects that the period in whichthe Vco is at the low level continues for a period from t1. This periodis set to be around 10 times to 100 times the switching period for therated load. For example, it is set to be 100 ms when the switchingperiod for the rated load is 10 ms. The inverting input terminal voltageVn of the comparator 225 a first decreases rapidly in the period oft1-t2 and gradually decreases thereafter. Upon detecting that the lowlevel of Vco continues for the period in t1-t2, the OFF period detectioncircuit 250 opens the switch element 226 with the output signal Vs ofthe OFF period detection circuit 250 and, at the same time, changes thereference voltage to a third intermediate voltage VpM with the outputsignal Vm of the OFF period detection circuit 250. The switch element226 is opened to cause the capacitive element 223 to be separated tocause Vn to be changed to a voltage V2×(R2/(R1+R2)) in which V2 isvoltage-divided by the third resistor and the fourth resistor. When thechanged Vn is higher than the reference voltage VpM, the low levelcontinues for Vco. These output signals Vs and Vm continue for a periodof t2-t3, which is sufficiently shorter than the above-described periodof t1-t2, and then Vs and Vm return at t3 to close the switch element226 again to cause the reference voltage to return to VpL.

In the period of t3-t4, in the same manner as t1-t2, the OFF perioddetection circuit 250 detects that the low level of Vco continues for aperiod, after which, in the period of t4-t5, Vs and Vm open the switchelement 226 to cause Vn to be changed to V2×(R2/(R1+R2)) and thereference voltage to be changed to VpM. Vn is still higher than VpM, sothat the low level of Vco is continued. Thereafter, the OFF perioddetection circuit 250 detects a period of t5-t6 to open the switchelement 226 with Vs and Vm at t6 to cause Vn to be changed toV2×(R2/(R1+R2)) and the reference voltage to be changed to VpM. Sincethe Vn decreases below VpM, the output Vco of the comparator 225 a isbrought to a high level from t6. The period in which the Vco is at thehigh level continues until t8 at which Vn rises up to VpH. At t7, whichis prior thereto, the Vs and Vm returns to close the switch element 226and bring the reference voltage to VpH.

Looking at the inverting input terminal voltage Vn of the comparator 225a immediately before t6, it is brought to be a voltage which is higherthan the reference voltage VpL, so it is seen that the Vco cannottransition to the high level at t6 unless the OFF period detectioncircuit 250 operates. Vn gradually decreases if the OFF period detectioncircuit 250 does not operate, causing the time taken for Vn to bebrought to a voltage which is lower than VpL to be long, and the periodin which Vco is at the low level to further continue for a long time. Ifthe OFF period detection circuit 250 operates, VpM is set as in Equation(23) below to cause Vco to transition to a low level with Equation (24)for the lower limit of the output voltage V2, causing the V2 to start toincrease.

$\begin{matrix}{V_{pM} = {\frac{V_{pH} + V_{pL}}{2} - {\frac{V_{pH} - V_{pL}}{2}\frac{R\; 2}{{R\; 1} + {R\; 2}}}}} & (23) \\{V_{2} = {{V_{pM}\frac{{R\; 1} + {R\; 2}}{R\; 2}} = {{\frac{V_{pH} + V_{pL}}{2}\frac{{R\; 1} + {R\; 2}}{R\; 2}} - \frac{V_{pH} - V_{pL}}{2}}}} & (24)\end{matrix}$

For the case of the heavy load such as in FIG. 12B, the lower limit ofthe output voltage V2 in Equation (24) is brought to a value which islower than Equation (8) for the average value of V2 for the case inwhich neither the switching frequency nor the output voltage V2decreases. The Vco transitions to the high level at the lower limit ofV2 to cause the voltage to increase by around VpH−VpL from the value inEquation (24), making it possible to make the average value of V2 inFIG. 12B and the average value of V2 when the switching frequencydecreases for the light load almost the same. Thus, the circuitaccording to the present embodiment may be connected to suppress thestatic load fluctuation Δ V2L of the output voltage.

As described above, the control circuit according to the seventhembodiment of the present invention includes third and fourth resistors221 and 222 that voltage-divide the output voltage of the switchingpower supply 201 a; a comparator 225 a in which the voltage-dividedvoltage is input to a first input terminal thereof and a referencevoltage Vp is input to a second input terminal thereof; and a controller230 which controls a switching transistor 205 a based on an outputsignal of the comparator 225 a, wherein the reference voltage Vp isbrought to a first voltage VpH when the output of the comparator 225 atakes a first level (high level) and is brought to a second voltage VpLwhen the output of the comparator 25 a takes a second level (low level),the third resistor 221 is connected between the output terminal positiveterminal 203 a of the switching power supply 201 a and the first inputterminal of the comparator 225 a; a serial connection is made of acapacitive element 223 and a switch element 226 in parallel with thethird resistor 221; and an OFF period detection circuit 250 whichdetects that the low level of the output of the comparator 225 acontinues longer than a period, wherein, when the low level of theoutput of the comparator 225 a continues longer than the period, the OFFperiod detection circuit 250 opens the switch element 226 and, at thesame time, causes the reference voltage to be brought to a third voltageVpM.

This may cause the load current to decrease to suppress the static loadfluctuation of the output voltage when the switching frequencydecreases.

Eighth Embodiment

FIG. 24 is a circuit diagram illustrating the configuration of aswitching power supply 201 b according to an eighth embodiment of thepresent invention. As one example, the switching power supply 201 bshown in FIG. 24 includes a pair of input terminals 202 a and 202 b(also called “an input terminal 202” when not specificallydistinguishing therebetween); a pair of output terminals 203 a and 203 b(also called “an output terminal 203” when not specificallydistinguishing between); a main circuit 204 b; and a control circuit 220b, and converts an input voltage (DC voltage) V1 input to the inputterminal 202 to an output voltage (DC voltage) V2 to output theconverted result from the output terminal 203 and controls the outputvoltage V2 to be at a predetermined target voltage. The switching powersupply 201 b inputs an input voltage V1 and an input current i1 to theinput terminal 2 to output the output voltage V2 and a load current i2from the output terminal 203.

In the same manner as the main circuit 204 a of the first embodiment,the main circuit 204 b is configured with a back-converter circuitscheme as one example of the switching power supply 201 b and outputs aninput voltage V1 input from the input terminal 202 to the output voltageV2 to output the converted result to the output terminal 203.

The control circuit 220 b includes a third resistor 221 and a fourthresistor 222 that voltage-divides the output voltage V2 of the switchingpower supply 201 b; a comparator 225 a in which a voltage-dividedvoltage Vn is input to a first inverting input terminal thereof and areference voltage Vp is input to a second non-inverting input terminalthereof; a controller 230 x which controls a switching transistor 205 abased on an output signal Vco of the comparator 225 a; the thirdresistor 221, which is connected between a positive terminal 203 a of anoutput terminal and a first inverting input terminal of the comparator225 a; and a series connection of a switch element 226 and a capacitiveelement 223 that are connected in parallel with this fourth resistor.The reference voltage Vp is brought to be a first high voltage VpH whenan output Vco of the comparator 225 a takes a first high level and to bea second low voltage VpL when an output Vco of the comparator 225 atakes a second low level. Moreover, the control circuit 220 b includesan OFF period detection circuit 250 which detects that the second levelof the output Vco of the comparator 225 a continues longer than aperiod, wherein, when the second level of the output of the comparator225 a continues longer than the period, it opens the switch element 226with an output signal Vs and, at the same time, brings the referencevoltage Vp to be a third intermediate voltage VpM with an output signalVm. A common ground G of the control circuit 220 b is connected to anegative terminal 203 b of the output terminal. A voltage of each ofsignal voltages Vn, Vp, and Vco is set to be a voltage with the commonground G as the reference.

Moreover, in the same manner as the fifth embodiment, a so-calledhysteresis comparator is provided in which a reference voltage Vp isbrought to be a first high voltage VpH when an output Vco of thecomparator 225 a takes a first high level and to be a second low voltageVpL when an output Vco of the comparator 225 a takes a second low level.As an example of this hysteresis comparator circuit scheme, in the samemanner as the fifth embodiment, the switching power supply 201 bincludes the comparator 225 a; a resistor 225 b which connects between anon-inverting input terminal and an output terminal of the comparator225 a; and a resistor 225 c and a constant voltage source 224 that areconnected in series between the non-inverting input terminal and thecommon ground G203 b; and a resistor 225 d which connects between thenon-inverting input terminal of the comparator 225 a and the OFF perioddetection circuit 250.

The controller 230 x includes a current detection element 230 d whichdetects a current iL which flows through a choke coil; a referencevoltage 230 c; a comparator 230 b which compares an output of thecurrent detection element 230 d and the reference voltage 230 c; and adrive device 230 a which drives the switching transistor 205 a based onan output Vco of the comparator 225 a and an output of the comparator230 b. The drive device 230 a turns on the switching transistor 205 awhen the output Vco of the comparator 225 a takes a first high level andthe output of the comparator 230 b takes a low level. The output of thecomparator 230 b is brought to the high level when turning on theswitching transistor 205 a causes the current iL which flows through thechoke coil 207 to be brought to at least a value. At this time, when theoutput Vco of the comparator 225 a continues at the first high level,the switching transistor 205 a is turned off for a period. Turning offthe switching transistor 205 a causes the current iL which flows thoughthe choke coil 207 to be lower than the value. When the output Vco ofthe comparator 225 a continues to be at the first high level eventhereafter, the switching transistor 205 a is turned on until thecurrent iL which flows through the choke coil 207 reaches at least thevalue again. When the output Vco of the comparator 225 a is at thesecond low level, the switching transistor 205 a is turned offregardless of the output of the comparator 230 b.

This causes switching to be performed a plurality of times during theperiod in which the output Vco of the comparator 225 a is at the firsthigh level.

Next, an operation of the control circuit 220 b is described. When thevoltage Vn in which the output voltage V2 is resistance-divideddecreases below the second voltage VpL, the comparator output Vco isbrought to a first high level to cause the controller 230 x to startdriving the switching transistor 205 a, so that the reference voltage isbrought to be the first voltage VpH. During the drive period of theswitching transistor 205, the current of the choke coil 207 increases tocause a current iL which is larger than a load current i2 to be suppliedfrom the choke coil 207 to the output capacitor 208 a, causing theoutput capacitor 208 a to be charged to raise the output voltage V2. Thecurrent iL which flows through the choke coil 207 being brought to be atleast a value which is larger than the load current turns off theswitching transistor 205 a for a period. Turning off the switchingtransistor 205 a causes the current iL which flows through the chokecoil 207 to be lower than a value. When the output Vco of the comparator225 a continues at the first high level even thereafter, the switchingtransistor 205 a is turned on until the current iL which flows throughthe choke coil 207 reaches at least a value again. This causes switchingto be performed a plurality of times during a period in which the outputVco of the comparator 225 a continues at the first high level. In thecontroller 230 x, when the voltage Vn in which the output voltage V2 isresistance-divided is brought to be higher than the first voltage VpH,the comparator output Vco is brought to a second low level to cause thedrive period to be completed and to be transitioned to a stop period. Atthis time, the reference voltage is brought to the second voltage VpL.In the stop period, the current iL from the choke coil 207 is brought to0 to cause discharging from the output capacitor 208 a by the loadcurrent i2 to cause the output voltage V2 to decrease. When the voltageVn in which the output voltage V2 is resistance-divided is again broughtto be lower than the second voltage VpL, the drive period is startedagain to bring the reference voltage to the first voltage VpH to switchthe switching transistor 205 a a plurality of times. Such an operationis repeated to cause the drive period and the stop period to becontrolled such that the resistance-divided voltage Vn is brought to bea value between the first voltage VpH and the second voltage VpL tocause the output voltage V2 to be controlled to the predetermined targetvoltage.

Here, the switching element 226 is closed when the switching frequencyis high, and a period in which the output Vco of the comparator 225 a isat the second low level. This causes the capacitive element 223 to beconnected to the third resistor 221 to cause the output voltage rippleto be suppressed. When the switching frequency decreases and a period inwhich the output Vco of the comparator 225 a is at the second low level,the switch element 226 and the reference voltage is brought to the thirdvoltage VpM. In the same manner as the seventh embodiment, this makes itpossible to suppress the static load fluctuation of the output voltage.

According to the eighth embodiment of the present invention,explanations for the ripple waveform of the output voltage V2 of theswitching power supply device 201 b is similar to those for FIGS. 15Aand 15B and the associated explanations according to the above-describedfourth embodiment of the present invention, so that they will be omittedhere.

As described above, for the control circuit of the eighth embodiment ofthe present invention, it is desirable to switch the switchingtransistor 205 a a plurality of times in a period in which the outputVco of the comparator 225 a is at the first level (high level). In thisway, as the on/off period of the switching transistor 205 a issufficiently shorter than the on/off period of the output Vco of thecomparator 225 a, the output Vco of the comparator 225 a being broughtto the first level (high level) causes the output voltage V2 to increaseimmediately, while the output Vco of the comparator 225 a being broughtto the second level (low level) causes the output voltage V2 todecrease, making it possible to suppress the output voltage ripple toaround the difference between the first voltage and the second voltage.

Moreover, for the control circuit according to the eighth embodiment ofthe present invention, it is desirable to turn off the switchingtransistor 205 a for a period when the current which flows through theswitching power supply 201 b is brought to at least a value. This causesthe current which flows through the switching power supply 201 b to bebrought to at least a value in the period in which the output Vco of thecomparator 225 a is at the first level (high level) to cause repeatingof the switching transistor 205 a being turned off for a period, makingit possible to cause performing of switching a plurality of times duringa period in which the output Vco of the comparator 225 a is at the firstlevel. This makes it possible to cause the output capacitor 208 a of theswitching power supply 201 b to be charged with a current which islarger than the load current in the period in which the output Vco ofthe comparator 225 a is at the first level, so that the output voltageV2 linearly increases at the same time as the beginning of the period inwhich the output voltage V2 is at the first level. The output capacitor208 a of the switching power supply 201 b is discharged with only theload current in the period in which the output Vco of the comparator 225a is at the second level (low level), so that the output voltage V2linearly decreases at the same time as the beginning of the period atthe second level. This makes it possible to suppress the output voltageripple to around the difference between the first voltage and the secondvoltage.

For the current which flows through the switching power supply, variousdetection methods are possible depending on the configuration of theswitching power supply. For example, the switching power supply 201 bshown in the eighth embodiment of the present invention may detect thecurrent of the switching transistor 205 a or the choke coil 207.Moreover, an insulation-type switching power supply may detect the coilcurrent of the transformer, while the current of a resonant capacitormay be detected when the resonant capacitor is present.

Ninth Embodiment

FIG. 25 is a circuit diagram illustrating a configuration of a switchingpower supply 201 c according to a ninth embodiment of the presentinvention. As one example, the switching power supply 201 c shown inFIG. 25 includes a pair of input terminals 202 a and 202 b (also called“an input terminal 202” when not specifically distinguishingtherebetween); a pair of output terminals 203 a and 203 b (also called“an output terminal 203” when not specifically distinguishing between);a main circuit 204 c; and a control circuit 220 cc, and converts aninput voltage (DC voltage) V1 input to the input terminal 202 to anoutput voltage (DC voltage) V2 to output the converted result from theoutput terminal 203 and controls the output voltage V2 to be at apredetermined target voltage. The switching power supply 201 c inputs aninput voltage V1 and an input current i1 to the input terminal 202 tooutput the output voltage V2 and a load current i2 from the outputterminal 203.

The main circuit 204 c includes a switching transistor 205 a, a parasitediode 205 b of the switching transistor 205 a, a diode 206, resonantinductors 209, 213, 214, and 217, resonant capacitors 210, 211, 212,215, and 216, an output capacitor 208 a, and an equivalent seriesresistor 208 b of the output capacitor 208 a. The switching power supply201 c, which is configured with a resonant converter circuit scheme asone example of the switching power supply, outputs an input voltage V1input from the input terminal 202 to the output voltage V2 to output theconverted result to the output terminal 203.

The control circuit 220 cc includes a third resistor 221 and a fourthresistor 222 that voltage-divides the output voltage V2 of the switchingpower supply 201 c; a comparator 225 a in which a voltage-dividedvoltage Vn is input to a first inverting input terminal thereof and areference voltage Vp is input to a second non-inverting input terminalthereof; a controller 230 v which controls a switching transistor 205 abased on an output signal Vco of the comparator 225 a; the thirdresistor 221, which is connected between a positive terminal 203 a of anoutput terminal and a first inverting input terminal of the comparator225 a; and a series connection of the capacitive element 223 and aswitch element 226 that are connected in parallel with the thirdresistor 221.

A reference voltage Vp is brought to be a first high voltage VpH when anoutput Vco of the comparator 225 a takes a first high level and isbrought to be a second low voltage VpL when an output Vco of thecomparator 225 a takes a second low level. Moreover, the control circuit220 c includes an OFF period detection circuit 250 which detects thatthe second level of the output Vco of the comparator 225 a continueslonger than a period, wherein, when the second level of the output Vcoof the comparator 225 a continues longer than the period, the OFF perioddetection circuit 250 opens the switch element 226 with the outputsignal Vs and, at the same time, brings the reference voltage Vp to athird intermediate voltage VpM with the output signal Vm. A commonground G of the control circuit 220 c is connected to a negativeterminal 203 b of the output terminal. A voltage of each of signalvoltages Vn, Vp, and Vco is set to be a voltage with the common ground Gas the reference.

Moreover, a so-called hysteresis comparator is provided in the samemanner as the fifth embodiment in which a reference voltage Vp isbrought to be a first high voltage VpH when an output Vco of thecomparator 225 a takes a first high level and is brought to be a secondlow voltage VpL when an output Vco of the comparator 225 a takes asecond low level. An exemplary hysteresis comparator circuit scheme, inthe same manner as the fifth embodiment, is configured with a hysteresiscomparator circuit scheme includes the comparator 225 a; a resistor 225b which connects between a non-inverting input terminal and an outputterminal of the comparator 225 a; a resistor 225 c and a constantvoltage source 224 that connects in series between the non-invertinginput terminal and the common ground G; and a resistor 225 d which isconnected between the non-inverting input terminal of the comparator 225a and the OFF period detection circuit 250.

The controller 230 y is configured with an oscillator 230 f whichoscillates at a frequency, and a drive device 230 e which drives theswitching transistor 205 a based on the output Vco of the comparator 225a and the output of the oscillator 230 f.

When the output Vco of the comparator 225 a is brought to the first highlevel, the drive device 230 e turns on and off the switching transistor205 a based on an output of the oscillator 230 f which oscillates at thefrequency. The inter-terminal voltage V5 of the switching transistor 205a increases from 0V by turning off of the switching transistor 205 a.The resonant capacitor 211 may be charged to cause the inter-terminalvoltage V5 to start increasing only after the switching transistor 205 ato be turned off completely to decrease the switching loss at the timeof turning off the switching transistor 205 a. Thereafter, by resonatingof the resonant inductors 209, 213, 214, 217, and the resonantcapacitors 210, 211, 212, 215, and 216, when the switching transistor205 a is turned off for a period the inter-terminal voltage V5 againreturns to 0V and a body diode 205 b of the switching transistor 205 ais turned on. Then, the switching transistor 205 a may be turned on tocause a so-called ZVS (zero voltage switching) to be performed, makingit possible to decrease a switching loss at the time of turning on theswitching transistor 205 a. When the output Vco of the comparator 225 acontinues at the first high level, a process is repeated such that theswitching transistor 205 a is turned off after being turned on during aperiod, making it possible to causing switching to be performed aplurality of times in the period in which the output Vco of thecomparator 225 a is at the first high level.

Next, an operation of the control circuit 220 cc is described. When thevoltage Vn in which the output voltage V2 is resistance-divided isbrought to be lower than the second voltage VpL, the output Vco of thecomparator 225 a is brought to the first high level, and the controller230 y starts driving of the switching transistor 205 a, and thereference voltage Vp is brought to the first voltage VpH. The switchingtransistor 205 a being turned on and off by the controller 230 y causesa current iD having a semi-sinusoidal wave with a peak value to flowfrom the diode 206. The time average of iD being larger than the loadcurrent i2 causes the output capacitor 208 a to be charged to cause theoutput voltage V2 to increase. In the controller 230 y, when the voltageVn in which the output voltage V2 is resistance-divided is brought to behigher than the first voltage VpH, the comparator output Vco is broughtto be at the second low level to cause the drive period to be completedto be transitioned to a stop period. Then, the reference voltage Vp isbrought to the second voltage VpL. In the stop period, the current iDfrom the diode 206 is brought to 0, causing discharging from the outputcapacitor 208 a by the load current i2 to cause the output voltage V2 todecrease. When the voltage Vn in which the output voltage V2 isresistance-divided is brought to be lower than the second voltage VpLagain, the drive period is started again to bring the reference voltageVp to the first voltage VpH to switch the switching transistor 205 a aplurality of times. Repeating this operation causes the drive period andthe stop period to be controlled such that the resistance-dividedvoltage Vn is brought to a value between the first voltage VpH and thesecond voltage VpL to cause the output voltage V2 to be controlled to apredetermined target voltage.

When the switching frequency is high and the period in which the outputVco of the comparator 225 a is at the second low level is short, theswitch element 226 is closed. In this way, the capacitive element 223 isconnected to the third resistor 221 to cause the output voltage rippleto be suppressed. The switching frequency decreasing and the period inwhich the output Vco of the comparator 225 a is at the second low levelincreasing opens the switching element 226 and the reference voltage tobe brought to a third voltage VpM. This makes it possible to suppressthe static load fluctuation of the output voltage in the same manner asthe seventh embodiment.

With reference to FIGS. 17A and 17B, a ripple waveform of the outputvoltage V2 of the switching power supply 1 c according to the ninthembodiment is described. The waveform of the load current i2 for a heavyload when the load current i2 is large is shown in FIG. 17A and thewaveform of the load current i2 for a light load when the load currenti2 is small is shown in FIG. 17B, along with the waveforms of thecurrent iD of the diode 206, the output Vco of the comparator 225 a, andthe output voltage V2. The capacitive element 223 may be connected todecrease the voltage ripple of the output voltage V2 from the product ofthe voltage-dividing ratio of (R1+R2)/R2 and the difference VpH−VpL ofthe first voltage and the second voltage to around the differenceVpH−VpL of the first voltage and the second voltage.

This is because the capacitive element 223 is connected to the thirdresistor 221 to stabilize the voltage Vr1 of the third resistor 221 toalmost a constant value. In this way, the voltage Vn of the invertinginput terminal of the comparator 225 a is brought to a value in which aVr1 is subtracted from the output voltage V2 to cause the voltage rippleof Vn to be equal to the voltage ripple of the output voltage V2. Thecontrol circuit 220 c controls the drive period and the stop period suchthat Vn is brought to be a value between VpL and VpH to control theoutput voltage V2 to a predetermined target voltage. Therefore, thevoltage ripple of Vn is brought to be equal to the difference VpH−VpLbetween the first voltage and the second voltage to also bring themagnitude of the voltage ripple of the output voltage V2 to VpH−VpL. AsV2=Vn+Vr1, the output voltage V2 is controlled to a value betweenVpH+Vr1 and VpL+Vr1.

When the output Vco of the comparator 225 a is brought to a first highlevel in to, the switching transistor 205 a is turned on and off by thecontroller 230 y to cause the current iD having a semi-sinusoidal wavewith a peak value to flow through the diode 206. The time average of iDis constant and larger than the load current i2, causing the outputcapacitor 208 a to undergo constant-current charging in the period oft0-t1 to cause the output voltage V2 to increase linearly. Moreover,during the period of t1-t2 in which the Vco is at the second low level,the current iD of the diode 206 is brought to 0. The output capacitor208 a undergoes constant current discharging only by the load current i2to cause the output voltage V2 to decrease linearly.

The output voltage V2 according to the ninth embodiment is similar tothe output voltage V2 in FIG. 17A and, in the same manner as the outputvoltage V2 according to the eighth embodiment in FIG. 24, is seldombrought to be larger than VpH+Vr1 or smaller than VpL+Vr1, so that themagnitude of the voltage ripple of the output voltage V2 is suppressedto a value which is closer to the hysteresis width of VpH−VpL. This isbecause, when the output Vco of the comparator 225 a is brought to thefirst high level at t0, iD reaches a current value of a constant currentwhich is larger than i2 in the time average value in a period which issufficiently shorter than the length of the period t0-t1 in which Vco isat the first high level to cause the output capacitor 208 a to undergoconstant current charging with iL−i2 to cause V2 to increase linearlyand, when the output Vco of the comparator 225 a is brought to thesecond low level at t1, iD is brought to 0 in a period which issufficiently shorter than the length of the period t1-t2 in which Vco isat the second low level to cause the output capacitor 208 a to undergoconstant current discharging with i2 to cause V2 to decrease linearly.

Moreover, in the same manner as the eighth embodiment, according to theninth embodiment, for causing switching to be performed a plurality oftimes in the period in which the output Vco of the comparator 225 a isat the first high level, charging and discharging of the outputcapacitor 208 a cause the voltage of the output capacitor 208 a at thetime of completion at t1 of the period in which Vco is at the first highlevel to be brought to be the maximum value of the voltage of the outputcapacitor 208 a. Moreover, the voltage of the output capacitor 208 a atthe time of completion at t2 of the period in which the output Vco ofthe comparator 225 a is at the second low level is brought to be theminimum value of the voltage of the output capacitor 208 a. Thus, evenwhen the voltage ripple of the equivalent series resistance 208 b is notpresent, the output voltage V2 may be controlled to be between VpH+Vr1and VpL+Vr1, causing the hysteresis control to operate correctly.Therefore, a capacitor with a small equivalent series resistance, suchas a ceramic capacitor, may be used for the output capacitor 208 a. Thecapacitor with the small equivalent series resistance may be used toachieve advantageous effects in reducing the dynamic load fluctuation ofthe output voltage V2 of the switching power supply 201 c and the highfrequency noise. Moreover, according to the seventh embodiment, theswitching frequency may be increased while suppressing the switchingloss of the switching transistor 205 a to decrease energy whichaccumulates in the resonant inductors 209, 213, 214, 217 and theresonant capacitors 210, 211, 212, 215, 216 that are used in the maincircuit 204 c of the resonant converter. The smaller the energyaccumulated in these resonant inductors and capacitors, the greater theability to cause the average value of the current iD of the diode 206 tofollow a value which is close to the load current i2 when the loadcurrent i2 changes rapidly, making it possible to suppress the dynamicload fluctuation of the output voltage. This makes it possible tosuppress the dynamic load fluctuation of the output voltage, the staticload fluctuation, and the output voltage ripple to around the differencebetween the first voltage and the second voltage.

With reference to FIG. 17B, a voltage ripple of the output voltage V2for the light load according to the ninth embodiment is described. Forthe light load, the period t4-t5 in which the output Vco of thecomparator 225 a is at the second low level increases to cause theswitching frequency to be low. In the same manner as the seventhembodiment (see FIG. 10B) and the eighth embodiment (see FIG. 17B), V2has a short rise period and, in the fall period thereof, causes theoutput capacitor 208 a to undergo constant current discharging over along time with a low load current i2, causing V2 to decrease linearly.FIG. 17B has the same waveforms as those in the seventh embodiment (seeFIG. 10B) and the eighth embodiment, so that, in the same manner as whatis described previously for the seventh and eighth embodiments, theaverage value of the inverting output terminal voltage Vn of thecomparator 225 a that is shown in FIG. 12A decreases from the centervalue to the same extent as the voltage ripple ΔVr1 of theinter-terminal voltage Vr1 of the third resistor 221. This results inthe static load fluctuation ΔV2L of the product of (R1+R2)/R2 and ΔVr1,so that it is necessary to open the switch element 226 to cause thecapacitive element 223 to be separated and the reference voltage to bebrought to a third voltage VpM when the switching frequency decreasesand the period in which the comparator output Vco is at the second lowlevel increases. This makes it possible to suppress the static loadfluctuation of the output voltage.

As described above, the control circuit 220 c according to the ninthembodiment of the present invention is a resonant converter in which themain circuit 204 c of the switching power supply 201 c includes aresonant circuit. In this way, the switching frequency may be increasedwhile suppressing the switching loss to decrease energy accumulated inthe inductors 209, 213, 214, 217 and capacitors 210, 211, 212, 215, 216.Therefore, the dynamic load fluctuation of the output voltage V2, thestatic load fluctuation, and the output voltage ripple may be suppressedto around the difference between the first voltage and the secondvoltage.

Tenth Embodiment

A tenth embodiment according to the present invention is described forthe configuration of a switching power supply device 201 d withreference to the drawings. The switching power supply device 201 d shownin FIG. 26 is an addition of a charging circuit 240 which charges thecapacitive element 223 at the time of activation to the configuration ofthe ninth embodiment shown in FIG. 25. The same charging circuit 240 maybe added also for the seventh and eighth embodiments. Describing theninth embodiment as an example, the switch element 226 is closed at thetime of activation, the capacitive element 223 is connected in parallelwith the third resistor 221, and the inter-terminal voltage of thecapacitive element 223 is 0V at the time of activation, so that theperiod at the time of activation of the switching power supply 201 cshown in FIG. 25 continues for a long time in which the inter-terminalvoltage Vr1 of the third resistor 221 is lower than the voltage of Vr1that is stable after completion of the activation. As the output voltageV2 is controlled to have the same value as that of the reference voltageVp, the inverting input terminal voltage Vn of the comparator 225 a iscontrolled such that V2=Vp+Vr1, so that the period in which V2 is alsolower than the predetermined target voltage continues for a long time.The predetermined target voltage for the output voltage V2 is Vp times(R1+R2)/R2, so that Vr1 then is Vp times R1/R2. For Vr1 to reach thevoltage of Vp×R1/R2 from 0V at the time of activation, the capacitiveelement 223 is charged along a route which goes through the capacitiveelement 223, the switch element 226, the fourth resistor 222, and thecommon ground G from the positive terminal 203 a of the output terminal.Taking into account the loss of the control circuit, resistance valuesR1 and R2 of the third resistor 221 and the fourth resistor 222 may notbe set to a small value (for example, less than or equal to 1 kΩ).Therefore, the charge current of the capacitive element 223 being smalland the static capacitance of the capacitive element 223 being largecause the period to reach the voltage Vp×R1/R2 from 0V at the time ofactivation to be longer.

The charging circuit 240 shown in FIG. 26 detects the output voltage V2to short-circuit the common ground G and the inverting input terminal ofthe comparator 225 a only in an activation period in which the outputvoltage V2 is also lower than the predetermined target voltage.Therefore, during the activation period, the capacitive element 223 ischarged with a large charge current along a route which passes throughthe capacitive element 223 and the charging circuit 240 from thepositive terminal 203 a of the output terminal. In this way, theactivation period for the output voltage V2 to reach the predeterminedtarget voltage from 0V may be shortened when the capacitive element 223is connected in parallel with the third resistor 221.

FIGS. 27A and 27B show embodiments of a so-called hysteresis comparatorin which a reference voltage Vp is brought to a first high voltage VpHwhen an output VcoH of the comparator 225 a takes a first high levelVcoH and to a second low voltage VpL when the output Vco of thecomparator 225 a takes a second low voltage VcoL, and to a thirdintermediate voltage VpM with a signal Vm from the OFF period detectioncircuit when the second low level of the output Vco of the comparator225 a continues for a long time. For the hysteresis comparator circuitscheme, a first specific example is shown in FIG. 27A, while a secondspecific example is shown in FIG. 27B.

The first specific example of the hysteresis comparator circuit schemeincludes a comparator 225 a; a resistor 225 b which connects between thenon-inverting input terminal and the output terminal of the comparator225 a; a constant voltage source 224 of a voltage V24 and a resistor 225c that connects serially between the common ground G and thenon-inverting terminal; and a resistor 225 d which connects between thenon-inverting input terminal and the OFF period detection circuit.

The reference voltage Vp when Vco is at the high level and Vm is at thelow level is brought to the high VpH, the reference voltage Vp when Vcois at the low level VcoL and Vm is at the low level is brought to thelow VpL, and the reference voltage Vp when Vco is at the low level andVm is at the high level is brought to the intermediate VpM.

The inverting input terminal voltage Vn of the comparator 225 a iscompared with a high VpH to determine whether Vco is changed from VcoHto VcoL in the period in which the output Vco of the comparator 225 a isat the first high level VcoH and the inverting input terminal voltage Vnof the comparator 225 a is compared with a low VpL to determine whetherVco is changed from VcoL to VcoH in the period in which the output Vcoof the comparator 225 a is at the second low level VcoL, so that theoutput Vco of the comparator 225 a is not changed and maintained to thefirst high level VcoH or the second low level VcoL when Vn takes anintermediate value of VpH and VpL. Vm is brought to the high level onlyin a period in which Vco is at the low level continues for a long timeto cause the OFF period detection circuit 250 to operate to cause theinput signal Vn to the comparator 225 a to be compared with VpM todetermine whether to change Vco from VcoL to VcoH.

A second specific example shown in FIG. 27B is a so-called windowcomparator which may realize a function equivalent to that of thehysteresis comparator according to the first specific example. Thesecond specific example includes a comparator 225 g, a comparator 225 h,a comparator 225 i, a constant voltage source 224 b of a voltage VpH, aresistor 225 j which voltage-divides the constant voltage source 224 b,a resistor 225 k, a resistor 225 m, an AND gate 225 n, an OR gate 225 o,and an SR flip-flop 225 f. The input signal Vn to the window comparatoris input to the non-inverting input terminal of the comparator 225 g,the inverting input terminal of the comparator 225 h, and the invertinginput terminal of the comparator 225 i; and the voltage VpH of theconstant voltage source 224 b is input to the inverting input terminalof the comparator 225 g. The voltage VpH of the constant voltage source224 b is voltage-divided by the resistor 225 j, the resistor 225 k, andthe resistor 225 m. The voltage VpM is input to the non-inverting inputterminal of the comparator 225 h, and the voltage VpL is input to thenon-inverting input terminal of the comparator 225 i.

The output of the comparator 225 i is brought to a high level to causethe SR flip-flop 225 f to be set to cause the output Vco of the SRflip-flop 225 f to be brought to the first high level VcoH, while theoutput of the comparator 225 g is brought to a high level to reset theSR flip-flop 225 f to be reset to bring the output Vco of the SRflip-flop 225 f to the second low level VcoL. In this way, in the periodin which the output Vco of the SR flip-flop 225 f is at the first highlevel VcoH, the input signal Vn into the window comparator is comparedwith the high VpH to determine whether to change Vco from VcoH to VcoL,and, in the period in which the output Vco of the SR flip-flop 225 f isat the second low level VcoL, the input signal Vn into the windowcomparator is compared with the low VpL to determine whether to changeVco from VcoL to VcoH, so that, when Vn takes an intermediate value ofVpH and VpL, Vco does not change and is maintained to the first highlevel VcoH or the second low level VcoL. Only when the period in whichVco is at the low level continues for a long time to cause the OFFperiod detection circuit to operate, Vm is brought to the high level tocause the input signal Vn into the window comparator to be compared withVpM to determine whether to change Vco from VcoL to VcoH.

Therefore, the window comparator shown in FIG. 27B may realize thefunction of the input signal Vn and the input signal Vm and the outputsignal Vco that are equivalent to the hysteresis comparator shown inFIG. 27A.

While a control circuit and a switching power supply device according toembodiments of the present invention have been described in theforegoing, they are not limited to descriptions of the above-describedembodiments, so that various types of variations are possible.

For example, functions equivalent to the hysteresis comparator and thewindow comparator according to the embodiments of the present inventionmay be embedded in integrated circuits, so that they are not limited tothe above-described embodiments.

Moreover, while the OFF period detection circuit 250 according to thepresent invention opens the switch element 226 and, at the same time,brings the reference voltage Vp to the third voltage VpM when the periodin which the second level VcoL of the output Vco of the comparator 225 acontinues longer than a period, timings of opening the switch element226 and, at the same time, causing the reference voltage Vp to bebrought to the third voltage VpM does not have be completelysimultaneous due to a delay in the circuit. The opening of the switchelement 226 may be prior to the timing at which the reference voltage Vpis caused to be brought from the second voltage VpL to the third voltageVpM, while closing the switch element again may be after the timing atwhich the reference voltage Vp is caused to be brought back from thethird voltage VpM to VpL or VpH.

Moreover, the switch element 226 may be a bipolar transistor or an FETfor example, wherein a parasite diode may be included inside the switchelement 226. When the OFF period detection circuit 250 opens the switchelement 226 to cause the capacitive element 223 to be separated from thethird resistor 221 in a case in which the second level VcoL of theoutput Vco of the comparator 225 a continues longer than a period, theswitch element 226 may have a function of blocking a discharge currentwhich flows from the negative terminal to the positive terminal (with adot) and may have a parasite diode of the switch element 226 in thedirection of passing a charge current which flows from the positiveterminal to the negative terminal of the capacitive element 223.

Eleventh Embodiment

FIG. 29 is a circuit diagram illustrating the configuration of theswitching power supply according to an eleventh embodiment of thepresent invention. As one example, the switching power supply 301 ashown in FIG. 29 includes a pair of input terminals 302 a and 302 b(also called “an input terminal 302” when not specificallydistinguishing therebetween); a pair of output terminals 303 a and 303 b(also called “an output terminal 303” when not specificallydistinguishing between); a main circuit 304; and a control circuit 310a, and converts an input voltage (DC voltage) V1 input to the inputterminal 302 to an output voltage (DC voltage) V2 to output theconverted result from the output terminal 303 and controls the outputvoltage V2 to be at a predetermined target voltage. The switching powersupply 301 a inputs an input voltage V1 and an input current i1 to theinput terminal 302 to output the output voltage V2 and a load current i2from the output terminal 303.

The main circuit 304 a includes a switching transistor 305, a diode 306,a choke coil 307, and an output capacitor 308. The switching powersupply 301 a, which is configured with a back converter circuit schemeas one example, outputs an input voltage V1 input from the inputterminal 302 to the output voltage V2 to output the converted result tothe output terminal 303.

The control circuit 310 a includes a third resistor 320 and a fourthresistor 321 that voltage-divides the output voltage V2 of the switchingpower supply 301 a; a comparator 322 in which a voltage-divided voltageVn is input to a first inverting input terminal thereof and a referencevoltage Vp is input to a second non-inverting input terminal thereof; acurrent detection circuit 309 which detects a current iL which flowsthrough the choke coil 307; and a controller 330 which controls aswitching transistor 305 based on a current value detected by thecurrent detection circuit 309 and an output signal Vco. The controlcircuit 310 a includes the third resistor 320, which is connectedbetween a positive terminal 303 a of an output terminal and a firstinverting input terminal of the comparator 322; and a capacitive element311 which is connected in parallel with the third resistor 320. A commonground G of the control circuit 310 a is connected to a negativeterminal 303 b of the output terminal. A voltage of each of signalvoltages Vn, Vp, and Vco is set to be a voltage with the common ground Gas the reference.

Moreover, a reference voltage Vp is brought to be a first high voltageVpH when an output Vco of the comparator 322 takes a first high leveland to be a second low voltage VpL when an output Vco of the comparator322 takes a second low level. The comparator 322 is a so-calledhysteresis comparator and, as an example of a circuit configurationwhich uses the hysteresis comparator, includes the comparator 322; aresistor 323 which connects between a non-inverting input terminal andan output terminal of the comparator 322; and a constant voltage source325 and a resistor 324 that are serially connected between thenon-inverting input terminal and the common ground G.

Next, an operation of the control circuit 310 a is described. When thevoltage Vn in which the output voltage V2 is resistance-divideddecreases below the second voltage VpL, the comparator output Vco isbrought to a first high level to cause the controller 330 to startdriving the switching transistor 305, so that the reference voltage isbrought to be the first voltage VpH. During the drive period of theswitching transistor 305, the current of the choke coil 307 increases tocause a current iL which is larger than a load current i2 to be suppliedfrom the choke coil 307 to the output capacitor 308, causing the outputcapacitor 308 to be charged to raise the output voltage V2. When thevoltage Vn in which the voltage V2 is resistance-divided is brought tobe higher than the first voltage VpH, the comparator output Vco isbrought to a second low level to cause the drive period to be completedand to be transitioned to a stop period. At this time, the referencevoltage is brought to be the second voltage VpL. In the stop period, theload current i2 is brought to be larger than the current iL from thechoke coil 307 to cause discharging from the output capacitor 308 tocause the output voltage V2 to decrease. When the voltage Vn in whichthe output voltage V2 is resistance-divided is again brought to be lowerthan the second voltage VpL, the drive period is started again to causethe reference voltage to be brought to the first voltage VpH. Such anoperation is repeated to cause the drive period and the stop period tobe controlled such that the resistance-divided voltage Vn is brought tobe a value between the first voltage VpH and the second voltage VpL tocause the output voltage V2 to be controlled to the predetermined targetvoltage.

Moreover, the capacitive element 311 is connected to the third resistor320 to stabilize the voltage across the third resistor 320 to almost aconstant value Vr1. This causes the voltage ripple of Vn of theinverting input terminal of the comparator 322 to be a value in which aVr1 is subtracted from the output voltage V2, so that the voltage rippleof Vn is brought to be equal to the voltage ripple of the output voltageV2. The control circuit 310 a controls the drive period and the stopperiod for Vn to take a value between VpL and VpH to control an outputvoltage V2 to a specified target voltage. Thus, the voltage ripple of Vnis brought to be equal to the difference between the first voltage andthe second voltage VpH−VpL to also cause the magnitude of the voltageripple of the output voltage V2 to be VpH−VpL.

Furthermore, the static capacitance of the capacitive element 311 may beset sufficiently large to cause Vr1 to be further stabilized and alsothe static load fluctuation of the output voltage V2 to also decrease.

However, when the static capacitance of the capacitive element 311 isincreased, it takes time for the voltage Vr1 thereacross to be chargedfrom 0V at the time of activation to a steady value. Assuming that theresistance value of the third resistor 320 is R1 and the resistance ofthe fourth resistor 321 is R2, Vr1=V2×R1/(R1+R2). Then, the outputvoltage V2 is controlled to be the target value Vp×(R1+R2)/R2 such as tocause Vn=V2−Vr1 to be equal to Vp. The period in which Vr1 at the timeof activation is lower than the steady value is also controlled such asto cause Vn=V2−Vr1 to be equal to Vp, causing V2 to be lower than thetarget value. When the period in which Vr1 is lower than the steadyvalue continues for a long time, the period in which the output voltagedoes not reach the target value continues for a long time.

Charging of the capacitive element 311 at the time of activation isperformed along a route which pass through the capacitive element 311and the fourth resistor 321 from the positive terminal 303 a of theoutput terminal and the larger the static capacitance of the capacitiveelement 311 and the larger the resistance value R2 of the fourthresistor 321, the more time it takes for the charging. The outputvoltage V2 is voltage-divided by the third resistor 320 and the fourthresistor 321, so that, when the output voltage V2 is high, R1 and R2need to be set with large resistance values to suppress the powerconsumption.

Then, for charging of the capacitive element 311 at the time ofactivation, a switch element 312 which is connected to parallel with thefourth resistor 321, and an output voltage detection circuit 313 areprovided. The output voltage detection circuit 313 is configured with acomparator 314 which is a second comparator, a second reference voltagesource 315 from which a reference voltage is input to the comparator314, a resistor 316 or a fifth resistor and a resistor 317 or a sixthresistor that voltage-divides the output voltage V2, wherein an input ismade from the voltage source 315 to the non-inverting input terminal ofthe comparator 314 and a voltage in which the output voltage V2 isvoltage-divided into the resistor 316 and the resistor 317 is input tothe inverting input terminal of the comparator 314. The output terminalof the comparator 314 drives the switch element 312. The output voltageV2 of the switching power supply 301 a is compared with a predeterminedvalue which is determined from the voltage source 315 and thevoltage-dividing ratio of the resistor 316 and the resistor 317 to closethe switch element 312 to cause the capacitive element 311 to be chargedwhen the output voltage V2 is low and to open the switch element 312when the output voltage V2 is high.

Moreover, during a period in which the switch element 312 is closed tocharge the capacitive element 311 rapidly, the output Vco of thecomparator 322 is brought to a first high level, so that the controller330 controls the switching transistor 305 to bring the current valuedetected by the current detection circuit 309 to a set maximum value.

In this way, at the time of activation at which the output voltage V2 islower than a predetermined value, the capacitive element 311 may becharged along a route which passes through the capacitive element 311and the switch element 312 from the positive terminal 303 a of theoutput terminal to rapidly perform charging even when the staticcapacitance of the capacitive element 311 is large, making it possibleto shorten the time until the output voltage V2 reaches the target valueat the time of activation.

Twelfth Embodiment

FIG. 30 is a circuit diagram illustrating the configuration of theswitching power supply 301 b according to a twelfth embodiment of thepresent invention. The switching power supply 301 b shown in FIG. 30 isdifferent in that the configuration of the output voltage detectioncircuit 313 is different from the switching power supply 301 a accordingto the eleventh embodiment of FIG. 29. The output voltage detectioncircuit 313 b is configured with a comparator 314, a voltage source 315,a resistor 316 and a resistor 317 that voltage-divide the output voltageV2, an RS flip-flop 318, and a signal source 319.

The voltage in which the output voltage V2 is voltage-divided by theresistor 316 and the resistor 317 is input to a non-inverting inputterminal of the comparator 314, the voltage source 315 is input to thenon-inverting input terminal of the comparator 314, and an output of thecomparator 314 is input to a reset input terminal of the RS flip-flop318. The signal source 319 is input to a reset input terminal of the RSflip-flop 318 and an output of the RS flip-flop 318 drives the switchelement 312. The signal source 319 becomes a high level only in a shortperiod immediately after activation to cause an output of the RSflip-flop 318 to drive the switch element 312. Therefore, the switchelement 312 is closed at the time of activation, so that the capacitanceelement 311 is charged rapidly in a manner similar to the eleventhembodiment of FIG. 29.

The output voltage V2 is low at the time of activation, so that avoltage in which the output voltage V2 is voltage-divided by theresistor 316 and 317 is lower than the voltage source 315, and an outputof the comparator 314 takes a low level. When the output voltage V2reaches a predetermined voltage, a voltage in which the output voltageV2 is voltage-divided by the resistor 316 and 317 is higher than thevoltage source 315 and an output of the comparator 314 becomes a highlevel to reset an output of the RS flip flop 318 to the low level andopen the switch element 312.

In this way, at the time of activation in which the output voltage V2 islower than a predetermined value, charging of the capacitive element 311is performed over a route which goes through the capacitive element 311and the switch element 312 from the output terminal positive terminal303 a to make it possible to rapidly charge even when the staticcapacitance of the capacitive element 311 is large, making it possibleto shorten the time until the output voltage V2 reaches a target valueat the time of activation.

Moreover, when the activation period is completed, the closing of theswitch element 312 does not occur, so that closing of the switch element312 to cause an increase of an output voltage ripple due to an increasein the output voltage V2 at the time of steady operation never occurs.Therefore, a predetermined value which compares the output voltage Vr1may be brought to a value which is close to the output voltage when thecapacitor voltage Vr1 becomes a steady value to make it possible toshorten the time for the output voltage V2 to reach the target value atthe time of activation.

Thirteenth Embodiment

FIG. 31 is a circuit diagram illustrating the configuration of theswitching power supply 301 c according to a thirteenth embodiment of thepresent invention. The switching power supply 301 c shown in FIG. 31 isdifferent from the switching power supply 301 b according to the twelfthembodiment of the present invention in FIG. 30 in the configuration ofthe control circuit 310 c. The control circuit 310 c is configured witha third resistor 320 and a fourth resistor 321 that voltage-divide theoutput voltage V2 of the switching power supply 301 c, an erroramplifier 326, a voltage source 325, a resistor 327, a capacitiveelement 311 c, a current detection circuit 309, a controller 330 c, aswitch element 312, and an output voltage detection circuit 313 b whichis the same as in FIG. 30.

A voltage which is voltage-divided by the third resistor 320 and thefourth resistor 321 is input to a first input terminal, which is anon-inverting input terminal, of the error amplifier 326, an input ismade from the voltage source 325 to the first input terminal, which isthe non-inverting input terminal, and the controller 330 c controls theswitching transistor 305 based on a current value which is detected bythe current detection circuit 309 and an output voltage level Va of theerror amplifier 326.

The resistor 327 and the capacitive element 311 c that are connectedbetween the output terminal and the inverting input terminal of theerror amplifier 326 cause an error between the output voltage V2 and thetarget voltage to be integrated. When a large time constant isintegrated, the static capacitance of the capacitive element 311 cincreases.

However, when the static capacitance of the capacitance element 311 cincreases, it takes time for the voltage V1 thereacross to be chargedfrom 0V at the time of activation to the steady value. The steady valueof V11 is Va0−Vap, which is a value in which the non-inverting inputterminal voltage Vp of the error amplifier 326 is subtracted from theoutput voltage level Va0 of the error amplifier 326 when the outputvoltage V2 stabilizes at the target voltage, and the charge current i11of the capacitive element 311 c then is 0. Then, assuming the resistancevalue of the first resistor 320 as R1, the resistance value of thesecond resistor 321 as R2, and the resistance value of the resistor 327as R3, the output voltage V2 is controlled to the target Vp×(R1+R2)/R2such as to cause Vn to be equal to Vp.

In the period in which V11 at the time of activation is lower than thesteady value, V2 is brought to be lower than the target value. As V11 islower than the steady value, a current of i11=(Va−Vp−V11)/R3 flows as acharge current i11 of the capacitive element 311 c and this current alsoflow through the second resistor 321, so that the output voltage V2 isbrought to be lower than the target value which is determined by thevoltage-dividing ratio by R1*i11=(Va−Vp−V11)R1/R3. The staticcapacitance of the capacitive element 311 c is large, so that, when aperiod in which V11 is lower than the steady value at the time ofactivation continues for a long time, a period in which the outputvoltage V2 does not reach the target value at the time of activationcontinues for a long time.

Then, to shorten the charging period of the capacitive element 311 c atthe time of activation, the switch element 312 and the output voltagedetection circuit 313 b that are connected in parallel with the secondresistor 321 are provided. In the same manner as the twelfth embodimentin FIG. 30, the output voltage detection circuit 313 b closes the switchelement 312 to cause the capacitive element 311 c to be charged at thetime of activation and the output voltage V2 is brought to be higherthan the predetermined voltage to open the switch element 312.

Moreover, in the period in which the switch element 312 is closed tocause the capacitive element 311 c to be charged rapidly, the controller330 c controls the switching transistor 305 such that the output Va ofthe error amplifier 326 is brought to be at a high voltage level tocause the current value detected by the current detection circuit 309 tobe brought to the set maximum value.

In this way, at the time of activation in which the output voltage V2 islower than a predetermined value, the capacitive element 311 c ischarged along a route which passes through the capacitive element 311 cand the switch element 312 from an output terminal of the erroramplifier 326 and may be charged rapidly even when the staticcapacitance of the capacitive element 311 c is large to cause the timefor the output voltage V2 to reach the target value at activation to beshortened.

While it has been described that the output voltage detection circuit313 b according to the thirteenth embodiment has the same configurationas the output voltage detection circuit 313 b according to the twelfthembodiment, it may be set to have the same configuration as the outputvoltage detection circuit 313 according to the eleventh embodiment.

While control circuits and switching power supplies according toembodiments of the present invention have been described in theforegoing, they are not limited to the above description of theembodiments, so that various types of variations are possible.

For example, while the switching power supplies are described with aback converter shown as an example, they are not limited thereto, sothat they may be applied to various types of switching power suppliessuch as a forward converter, a push-pull converter, etc.

A control circuit according to one aspect of the present invention isconfigured to control a switching element of a switching power supply,the control circuit including: a comparator having a first inputterminal configured to receive an output voltage of the switching powersupply, the comparator having a second input terminal that isconnectable to a positive terminal of a reference voltage source, thecomparator having an output, the output brings the reference voltage toa first voltage while the output signal takes a first voltage level, theoutput brings the reference voltage to a second voltage while the outputsignal takes a second voltage level; and a constant voltage sourcehaving a positive terminal connected to a negative terminal of thereference voltage source and a ground of the comparator. This makes itpossible to suppress an output voltage ripple to a width between thefirst voltage and the second voltage.

Moreover, the control circuit may further include a level shift circuitconfigured to be between the output of the comparator and an input of acontroller, the level shift circuit being configured to change the firstvoltage level or the second voltage level that is transmitted from theoutput of the comparator, the level shift circuit configured to supplythe controller with the first voltage level changed or the secondvoltage changed, wherein the controller is configured to control theswitching element based on the output from the level shift circuit. Thismakes it possible to suppress the output voltage ripple to the widthbetween the first voltage and the second voltage and perform shifting toa voltage level which drives the control circuit.

Furthermore, the level-shift circuit may include a Zener diodeconfigured to be between a Zener diode between the output of thecomparator and the input of the controller, and a first resistorconfigured to be between the input of the controller and a commonground. In this way, shifting to a voltage level which drives thecontrol circuit may be made with a simple circuit.

Moreover, the constant voltage source may include a capacitor. Thismakes it possible to suppress the voltage ripple of the constant voltagesource with the capacitor.

Furthermore, the controller may be configured perform switching aplurality of times in a period in which the output of the comparator isat the first voltage level and stop switching in a period in which theoutput of the comparator is at the second voltage level. In this way, anon-off period of the switching element is sufficiently shorter than aburst period which is a period of on/off of the comparator output.Therefore, the output voltage rises when the comparator output becomesthe first level, and the output voltage falls immediately when thecomparator output reaches the second level to cause the output voltageto fall, making it possible to suppress the output voltage ripple to thewidth between the first voltage and the second voltage.

Moreover, the controller may be configured to turn the switching elementoff when a current which flows through the switching power supplybecomes at least a predetermined value. In this way, the output voltageripple may be suppressed to a width between the first voltage and thesecond voltage.

Furthermore, the switching power supply may include a resonantconverter. In this way, the switching frequency may be raised whilesuppressing the loss, making it possible to decrease energy accumulatedin the inductor and capacitor used in the resonant converter. Thus, inaddition to the output ripple, output voltage fluctuations to rapid loadchanges to be suppressed to the hysteresis width.

Moreover, according to another aspect of the present invention, acontrol circuit is configured to control a switching element of aswitching power supply having an output terminal connected to a positiveterminal of a constant voltage source, the control circuit including: acomparator having a first input terminal that is connectable to anegative terminal of the constant voltage source and the comparatorhaving a second input terminal that is connectable to a referencevoltage source, the comparator having an output, the output brings thereference voltage to a first voltage while the output signal takes afirst voltage level, the output brings the reference voltage to a secondvoltage while the output signal takes a second voltage level; and, and acontroller configured to control the switching element based on theoutput of the comparator.

Furthermore, the constant voltage source may include a Zener diode thatis connectable between the output terminal of the switching power supplyand the first input terminal of the comparator, and a second resistorthat is connectable between the first input terminal of the comparatorand the common ground. In this way, a simple constant voltage sourcewith the small number of components may be made.

Moreover, the constant voltage source may include a shunt regulatorconfigured to adjust a setting voltage. This may secure an accurateconstant voltage source to suppress the output voltage ripple to a widthbetween a first voltage and a second voltage.

According to a further aspect of the present invention, a controlcircuit configured to control a switching transistor of a switchingpower supply is provided, the control circuit including: a thirdresistor and a fourth resistor that are connected to each other tovoltage-divide an output voltage of the switching power supply; acomparator having a first input terminal configured to receive theoutput voltage voltage-divided by the third resistor and the fourthresistor, the comparator having a second input terminal configured toreceive a reference voltage; and a controller configured to control theswitching transistor based on an output signal of the comparator,wherein the control circuit is configured to bring the reference voltageto a first voltage while the output signal of the comparator takes afirst voltage level and the control circuit is configured to bring thereference voltage to a second voltage while the output signal of thecomparator takes a second voltage level, and the third resistor isconnectable between a positive terminal of an output terminal of theswitching power supply and the first input terminal of the comparator;the control circuit further including; a capacitive element that isconnectable to the third resistor in parallel with each other; and,wherein, where a resistance value of the first resistance is set to R1,a resistance value of the fourth resistor is set to R2, a minimumswitching frequency of the switching transistor is set to Fmin, and astatic capacitance of the capacitive element is set to C1, Equation (1)

$\begin{matrix}{{C\; 1} \geqq \frac{{R\; 1} + {R\; 2}}{2\pi \; F_{\min}R\; 2^{2}}} & (1)\end{matrix}$

is met.

In this way, static load fluctuations of the output voltage when theload current decrease to bring the minimum switching frequency Fmin toaround the difference between the first voltage and the second voltage.

The control circuit according to a still further aspect of the presentinvention may include a controller configured to switch the switchingtransistor a plurality of times when the output of the comparator is atthe first voltage level. In this way, as the period of on/off of theswitching transistor is sufficiently shorter than the period of theon/off period of the comparator output, the output voltage risesimmediately after the comparator output becomes a first level and theoutput voltage drops immediately after the comparator output becomes asecond level, so that the output voltage ripple may be suppressed toaround the difference between the first voltage and the second voltage.

Moreover, in the control circuit, the controller may be configured toturn the switching transistor off when a current which flows through theswitching power supply becomes a predetermined value. In this way, acurrent which flows through the switching power supply becomes a valuewhen the output of the comparator is at the first level to turn theswitching transistor off in the period, making it possible to switch aplurality of times when the output of the comparator is at the firstlevel. Thus, an output capacitor of the switching power supply may becharged with a current which is larger than the load current while thecomparator output takes a first voltage level, so that the outputvoltage linearly increases simultaneously with the beginning of thefirst level. While the comparator output takes a second voltage level,the output capacitor of the switching power supply apparatus is chargedby only the load current, so that the output voltage decreases linearlywith the beginning of the second level. In this way, the output voltageripple may be suppressed to around the difference between the firstvoltage and the second voltage.

Furthermore, the switching power supply according to a yet furtheraspect of the present invention may be a resonant converter. In thisway, the switching frequency may be increased while suppressing theswitching loss to decrease energy accumulated in the inductor andcapacitor used for the resonance converter. Thus, dynamic loadfluctuations of the output voltage, static load fluctuations, and outputvoltage ripples may be suppressed to around the difference between thefirst voltage and the second voltage.

Furthermore, the control circuit according to a still further aspect ofthe present invention may include a charging circuit configured tocharge the capacitive element when the switching power supply isactivated. In this way, the capacitive element may be rapidly charged toprevent activation from delaying.

According to a further aspect of the present invention, a controlcircuit configured to control a switching transistor of a switchingpower supply is provided, the control circuit including: a thirdresistor and a fourth resistor that are connected to each other tovoltage-divide an output voltage of the switching power supply; acomparator having a first input terminal configured to receive theoutput voltage voltage-divided by the third resistor and the fourthresistor, the comparator having a second input terminal configured toreceive a reference voltage; and a controller configured to control theswitching transistor based on an output signal of the comparator,wherein the control circuit is configured to bring the reference voltageto a first voltage while the output signal of the comparator takes afirst voltage level and the control circuit is configured to bring thereference voltage to a second voltage while the output signal of thecomparator takes a second voltage level, and the third resistor isconnectable between a positive terminal of an output terminal of theswitching power supply and the first input terminal of the comparator;the control circuit further including: a capacitive element and a switchelement that are connected in parallel with the third resistor and aremutually connected in series; and an OFF period detection circuitconfigured to detect that the output of the comparator continually takesthe second voltage level in a period which is longer than apredetermined period, wherein the OFF period detection circuit isconfigured to open the switch element and bring the reference voltage toa third voltage when the OFF period detection circuit detects that theoutput of the comparator continually takes the second voltage level inthe period which is longer than the predetermined period.

The OFF period detection circuit according to a still further aspect ofthe present invention may be configured to close the switch element andbring the reference voltage to the first voltage when the OFF perioddetection circuit detects that the output of the comparator takes thefirst voltage level.

In this way, when determining the timing at which an output of thecomparator again becomes a second level after the output of thecomparator becomes the first level as a result of the output voltagevoltage-divided at the first input terminal and a third voltage of thesecond input terminal with the OFF period detection circuit functioning,returning to a normal operation when the OFF period detection circuit isnot functioning is made possible.

Moreover, in the OFF period detection circuit according to a furtheraspect of the present invention, the OFF period detection circuit isconfigured to open the switch element in the period which is shorterthan the predetermined period and bring the reference voltage to thethird voltage when the OFF period detection circuit detects that theoutput of the comparator continually takes the second voltage level inthe period which is longer than the predetermined period; and, after theperiod which is shorter than the predetermined period, to close theswitch element, and bring the reference voltage to the first voltagewhen the output of the comparator is at the first voltage level and tothe second voltage when the output of the comparator is at the secondvoltage level.

In this way, when the difference between the output voltagevoltage-divided at the first input terminal and the third voltage at thesecond input terminal is small, a comparison period may be shortened toprevent error operation due to noise.

Moreover, the control circuit according to a further aspect of thepresent invention may include a controller which is configured to switchthe switching transistor a plurality of times when the output of thecomparator is at the first voltage level. In this way, when the periodof the on/off of the switching transistor is sufficiently shorter thanthe period of on/off of the comparator output, an output voltageincrease immediately when the comparator output reaches the first leveland an output voltage decreases immediately when the comparator outputreaches the second level, making it possible to suppress the outputvoltage ripple to around the difference between the first voltage andthe second voltage.

Furthermore, the controller may be configured to turn the switchingtransistor off when a current which flows through the switching powersupply becomes the predetermined value. In this way, a process isrepeated in which the current which flows through the switching powersupply becomes the value while the comparator output is at the firstlevel to turn off the switching transistor for a period, making itpossible to perform switching a plurality of times while the comparatoroutput is at the first level. Therefore, the output capacitor of theswitching power supply may be charged with a current which is largerthan a load current when a comparator output takes a first level, sothat the output voltage linearly rises simultaneously with the start ofthe first level. The comparator output is at the second level, theoutput capacitor of the switching power supply is discharged with onlythe load current, so that the output voltage linearly decreasessimultaneously with the start of the period when the output voltage isat the second level. In this way, the output voltage ripple may besuppressed to around the difference between the first voltage and thesecond voltage. Therefore, the dynamic load fluctuations of the outputvoltage, static load fluctuations, and the output voltage ripple may besuppressed to around the difference between the first voltage and thesecond voltage.

Furthermore, the control circuit according to a further aspect of thepresent invention may include a charging circuit configured to chargethe capacitive element when the switching power supply is activated. Inthis way, the capacitance element may be rapidly charged to preventactivation from being delayed.

A control circuit according to a further aspect of the present inventionmay include a capacitive element; a switch element that is connectableto the capacitive element; and an output voltage detection circuitconfigured to compare an output voltage of the switching power supplywith a predetermined value, close the switch element to charge thecapacitive element when the output voltage is lower than thepredetermined value, and open the switch element when the output voltageis higher than the predetermined value.

In this way, when a capacitor of a large capacitance is used for acontrol circuit of a switching power supply, charging is performed in ashort time by the switch element from 0V to the steady value at the timeof activation, no time is taken to the output voltage reaching thetarget value at the time of activation.

A control circuit according to a further embodiment of the presentinvention includes a capacitive element; a switch element that isconnectable to the capacitive element; and an output voltage detectioncircuit configured to close the switch element to charge the capacitiveelement when the switching power supply is activated, and compare anoutput voltage of the switching power supply with a predetermined valueto open the switch element when the output voltage is higher than thepredetermined value.

In this way, for using a capacitor with a large static capacitance forthe control circuit of the switching power supply, charging is performedin a short time by the switching element from 0V to the steady value atthe time of activation, so that no time is taken for the output voltageto reach the target value at the time of activation. Moreover, when theactivation period is completed, closing of the switch element does notoccur again, so that, at the time of steady operation, the outputvoltage is decreased to close the switch element, so that the outputvoltage ripple does not increase. Therefore, a predetermined value withwhich to compare the output voltage may be set high, making it possibleto make the value close to an output voltage when the capacitor voltagebecomes a steady value to shorten the time until the output voltagereaches the target value at the time of activation.

The control circuit according to a further aspect of the presentinvention may have a capacitive element connectable to an outputterminal of the switching power supply. This makes it possible tostabilize the output voltage of the switching power supply by thecapacitive element.

The control circuit according to a further embodiment of the presentinvention may be provided, further including a third resistor and afourth resistor that are connected to voltage-divide an output voltageof the switching power supply; a comparator having a first inputterminal configured to receive the output voltage voltage-divided by thethird resistor and the fourth resistor, the comparator having a secondinput terminal configured to receive a reference voltage; and acontroller configured to control a switching transistor based on anoutput signal of the comparator, wherein the reference voltage input tothe second terminal of the comparator becomes a first voltage when anoutput of the comparator takes a first voltage level and a secondvoltage when the output of the comparator takes a second voltage level,and the third resistor is connectable between a positive terminal of theoutput terminal of the switching power supply and the first inputterminal of the comparator, and the control circuit has the capacitiveelement connected in parallel with the third resistor.

In this way, the capacitive element connected in parallel with thevoltage-dividing resistance may be rapidly charged at the time ofactivation to shorten the time for the output voltage to reach thetarget value at the time of activation.

The control circuit according to a further embodiment of the presentinvention may be provided, further including a third resistor and afourth resistor that are configured to voltage-divide an output voltageof the switching power supply; an error amplifier having a first inputterminal configured to receive the output voltage voltage-divided by thethird resistor and the fourth resistor, the error amplifier having asecond input terminal configured to receive a reference voltage; and acontroller configured to control a switching transistor based on anoutput voltage of the error amplifier, wherein the capacitive element isinsertable between an output terminal of the error amplifier and thefirst input terminal of the error amplifier.

This makes it possible for a capacitance element for integration inwhich an output of the error amplifier is fed back to an input toshorten the time for the output voltage to reach the target value at thetime of activation.

In a control circuit according to a further aspect of the presentinvention, the output voltage detection circuit may include an outputvoltage detection circuit which may include a fifth resistor and a sixthresistor configured to voltage-divide an output voltage of the switchingpower supply; a second comparator; and a reference voltage sourceconfigured to supply the second comparator with the reference voltage.

This makes it possible to shorten the time for the output voltage toreach the target value at the time of activation to configure the outputvoltage detection circuit with a simple configuration.

A control circuit according to a further embodiment of the presentinvention may further include a flip-flop circuit and a signal sourcethat is connectable to the flip-flop circuit.

This makes it possible to shorten the time for the output voltage toreach the target value at the time of activation to configure the outputvoltage detection circuit with a simple configuration.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the scope of the present invention. Accordingly, theinvention is not to be considered as being limited by the foregoingdescription, and is only limited by the scope of the appended claims.

What is claimed is:
 1. A control circuit configured to control aswitching element of a switching power supply, the control circuitcomprising: a comparator having a first input terminal configured toreceive an output voltage of the switching power supply, the comparatorhaving a second input terminal that is connectable to a positiveterminal of a reference voltage source, the comparator having an output,the output brings the reference voltage to a first voltage while theoutput signal takes a first voltage level, the output brings thereference voltage to a second voltage while the output signal takes asecond voltage level; and a constant voltage source having a positiveterminal connected to a negative terminal of the reference voltagesource and a ground of the comparator.
 2. The control circuit as claimedin claim 1, further comprising: a level shift circuit configured to bebetween the output of the comparator and an input of a controller, thelevel shift circuit being configured to change the first voltage levelor the second voltage level that is transmitted from the output of thecomparator, the level shift circuit configured to supply the controllerwith the first voltage level changed or the second voltage changed,wherein the controller is configured to control the switching elementbased on the output from the level shift circuit.
 3. The control circuitas claimed in claim 2, wherein the level shift circuit comprises: aZener diode configured to be between the output of the comparator andthe input of the controller, and a first resistor configured to bebetween the input of the controller and a common ground.
 4. The controlcircuit as claimed in claim 1, wherein the constant voltage sourcecomprises a capacitor.
 5. The control circuit as claimed in claim 2,wherein the controller is configured to perform switching a plurality oftimes in a period in which the output of the comparator is at the firstvoltage level and stop switching in a period in which the output of thecomparator is at the second voltage level.
 6. The control circuit asclaimed in claim 2, wherein the controller is configured to turn theswitching element off when a current which flows through the switchingpower supply becomes at least a predetermined value.
 7. The controlcircuit as claimed in claim 1, wherein the switching power supplycomprises a resonant converter.
 8. A control circuit configured tocontrol a switching element of a switching power supply having an outputterminal connected to a positive terminal of a constant voltage source,the control circuit comprising: a comparator having a first inputterminal that is connectable to a negative terminal of the constantvoltage source and the comparator having a second input terminal that isconnectable to a reference voltage source, the comparator having anoutput, the output brings the reference voltage to a first voltage whilethe output signal takes a first voltage level, the output brings thereference voltage to a second voltage while the output signal takes asecond voltage level; and a controller configured to control theswitching element based on the output of the comparator.
 9. The controlcircuit as claimed in claim 8, wherein the constant voltage sourcecomprises a Zener diode that is connectable between the output terminalof the switching power supply and the first input terminal of thecomparator, and a second resistor that is connectable between the firstinput terminal of the comparator and the common ground.
 10. The controlcircuit as claimed in claim 8, wherein the constant voltage sourcecomprises a shunt regulator configured to adjust a setting voltage. 11.The control circuit as claimed in claim 8, wherein the controller isconfigured to perform switching a plurality of times in a period inwhich the output of the comparator takes a first voltage level and stopswitching in a period in which the output of the comparator is at thesecond voltage level.
 12. The control circuit as claimed in claim 8,wherein the controller is configured to turn the switching element offwhen a current which flows through the switching power supply becomes atleast a predetermined value.
 13. The control circuit as claimed in claim8, wherein the switching power supply comprises a resonant converter.14. A switching power supply comprising: a control circuit configured tocontrol a switching element of a switching power supply, the controlcircuit comprising: a comparator having a first input terminalconfigured to receive an output voltage of the switching power supply,the comparator having a second input terminal that is connectable to apositive terminal of a reference voltage source, the comparator havingan output, the output brings the reference voltage to a first voltagewhile the output signal takes a first voltage level, the output bringsthe reference voltage to a second voltage while the output signal takesa second voltage level; and a constant voltage source having a positiveterminal connected to a negative terminal of the reference voltagesource and a ground of the comparator.
 15. A switching power supplycomprising: a control circuit configured to control a switching elementof a switching power supply having an output terminal connected to apositive terminal of a constant voltage source, the control circuitcomprising: a comparator having a first input terminal that isconnectable to a negative terminal of the constant voltage source andthe comparator having a second input terminal that is connectable to areference voltage source, the comparator having an output, the outputbrings the reference voltage to a first voltage while the output signaltakes a first voltage level, the output brings the reference voltage toa second voltage while the output signal takes a second voltage level;and a controller configured to control the switching element based onthe output of the comparator.
 16. A control circuit configured tocontrol a switching transistor of a switching power supply, the controlcircuit comprising: a third resistor and a fourth resistor that areconnected to each other to voltage-divide an output voltage of theswitching power supply; a comparator having a first input terminalconfigured to receive the output voltage voltage-divided by the thirdresistor and the fourth resistor, the comparator having a second inputterminal configured to receive a reference voltage; and a controllerconfigured to control the switching transistor based on an output signalof the comparator, wherein the control circuit is configured to bringthe reference voltage to a first voltage while the output signal of thecomparator takes a first voltage level, and the control circuit isconfigured to bring the reference voltage to a second voltage while theoutput signal of the comparator takes a second voltage level, and thethird resistor is connectable between a positive terminal of an outputterminal of the switching power supply and the first input terminal ofthe comparator; the control circuit further comprising: a capacitiveelement that is connectable to the third resistor in parallel with eachother, wherein, where a resistance value of the third resistor is R1, aresistance value of the fourth resistor is R2, a minimum switchingfrequency of the switching transistor is Fmin, and a static capacitanceof the capacitive element is C1, Equation (1) $\begin{matrix}{{C\; 1} \geqq \frac{{R\; 1} + {R\; 2}}{2\pi \; F_{\min}R\; 2^{2}}} & (1)\end{matrix}$ is met.
 17. The control circuit as claimed in claim 16,wherein the controller is configured to switch the switching transistora plurality of times when the output of the comparator is at the firstvoltage level.
 18. The control circuit as claimed in claim 16, whereinthe controller is configured to turn the switching transistor off when acurrent which flows through the switching power supply becomes apredetermined value.
 19. The control circuit as claimed in claim 16,wherein the switching power supply comprises a resonant converter. 20.The control circuit as claimed in claim 16, wherein the control circuitcomprises a charging circuit configured to charge the capacitive elementwhen the switching power supply is activated.
 21. A switching powersupply comprising: a control circuit configured to control a switchingtransistor of a switching power supply, the control circuit comprising:a third resistor and a fourth resistor that are connected to each otherto voltage-divide an output voltage of the switching power supply; acomparator having a first input terminal configured to receive theoutput voltage voltage-divided by the third resistor and the fourthresistor, the comparator having a second input terminal configured toreceive a reference voltage; and a controller configured to control theswitching transistor based on an output signal of the comparator,wherein the control circuit is configured to bring the reference voltageto a first voltage while the output signal of the comparator takes afirst voltage level, and the control circuit is configured to bring thereference voltage to a second voltage while the output signal of thecomparator takes a second voltage level, and the third resistor isconnectable between a positive terminal of an output terminal of theswitching power supply and the first input terminal of the comparator;the control circuit further comprising: a capacitive element that isconnectable to the third resistor in parallel with each other, wherein,where a resistance value of the third resistor is R1, a resistance valueof the fourth resistor is R2, a minimum switching frequency of theswitching transistor is Fmin, and a static capacitance of the capacitiveelement is C1, Equation (1) $\begin{matrix}{{C\; 1} \geqq \frac{{R\; 1} + {R\; 2}}{2\pi \; F_{\min}R\; 2^{2}}} & (1)\end{matrix}$ is met.
 22. A control circuit configured to control aswitching transistor of a switching power supply, the control circuitcomprising: a third resistor and a fourth resistor that are connected tovoltage-divide an output voltage of the switching power supply; acomparator having a first input terminal configured to receive theoutput voltage voltage-divided by the third resistor and the fourthresistor, the comparator having a second input terminal configured toreceive a reference voltage; and a controller configured to control theswitching transistor based on an output signal of the comparator,wherein the control circuit is configured to bring the reference voltageto a first voltage while the output signal of the comparator takes afirst voltage level and the control circuit is configured to bring thereference voltage to a second voltage while the output signal of thecomparator takes a second voltage level, and the third resistor isconnectable between a positive terminal of an output terminal of theswitching power supply and the first input terminal of the comparator;the control circuit further comprising: a capacitive element and aswitch element that are connected in parallel with the third resistorand are mutually connected in series; and an OFF period detectioncircuit configured to detect that the output of the comparatorcontinually takes the second voltage level in a period which is longerthan a predetermined period, wherein the OFF period detection circuit isconfigured to open the switch element and bring the reference voltage toa third voltage when the OFF period detection circuit detects that theoutput of the comparator continually takes the second voltage level inthe period which is longer than the predetermined period.
 23. Thecontrol circuit as claimed in claim 22, wherein the OFF period detectioncircuit is configured to close the switch element and bring thereference voltage to the first voltage when the OFF period detectioncircuit detects that the output of the comparator takes the firstvoltage level.
 24. The control circuit as claimed in claim 22, wherein,the OFF period detection circuit is configured to open the switchelement in a period which is shorter than the predetermined period andbring the reference voltage to the third voltage when the OFF perioddetection circuit detects that the output of the comparator continuallytakes the second voltage level in the period which is longer than thepredetermined period; and, after the period which is shorter than thepredetermined period, to close the switch element, and bring thereference voltage to the first voltage when the output of the comparatoris at the first voltage level and to the second voltage when the outputof the comparator is at the second voltage level.
 25. The controlcircuit as claimed in claim 22, wherein the controller is configured toswitch the switching transistor a plurality of times when the output ofthe comparator is at the first voltage level.
 26. The control circuit asclaimed in claim 22, wherein the controller is configured to turn theswitching transistor off when a current which flows through theswitching power supply becomes the predetermined value.
 27. The controlcircuit as claimed in claim 22, wherein the switching power supplycomprises a resonant converter.
 28. The control circuit as claimed inclaim 22, further comprising: a charging circuit configured to chargethe capacitive element when the switching power supply is activated. 29.A switching power supply comprising: a control circuit configured tocontrol a switching transistor of a switching power supply, the controlcircuit comprising: a third resistor and a fourth resistor that areconnected to voltage-divide an output voltage of the switching powersupply; a comparator having a first input terminal configured to receivethe output voltage voltage-divided by the third resistor and the fourthresistor, the comparator having a second input terminal configured toreceive a reference voltage; and a controller configured to control theswitching transistor based on an output signal of the comparator,wherein the control circuit is configured to bring the reference voltageto a first voltage while the output signal of the comparator takes afirst voltage level and the control circuit is configured to bring thereference voltage to a second voltage while the output signal of thecomparator takes a second voltage level, and the third resistor isconnectable between a positive terminal of an output terminal of theswitching power supply and the first input terminal of the comparator;the control circuit further comprising: a capacitive element and aswitch element that are connected in parallel with the third resistorand are mutually connected in series; and an OFF period detectioncircuit configured to detect that the output of the comparatorcontinually takes the second voltage level in a period which is longerthan a predetermined period, wherein the OFF period detection circuit isconfigured to open the switch element and bring the reference voltage toa third voltage when the OFF period detection circuit detects that theoutput of the comparator continually takes the second voltage level inthe period which is longer than the predetermined period.
 30. A controlcircuit of a switching power supply, the control circuit comprising: acapacitive element; a switch element that is connectable to thecapacitive element; and an output voltage detection circuit configuredto compare an output voltage of the switching power supply with apredetermined value, close the switch element to charge the capacitiveelement when the output voltage is lower than the predetermined value,and open the switch element when the output voltage is higher than thepredetermined value.
 31. A control circuit of a switching power supply,the control circuit comprising: a capacitive element; a switch elementthat is connectable to the capacitive element; and an output voltagedetection circuit configured to close the switch element to charge thecapacitive element when the switching power supply is activated, andcompare an output voltage of the switching power supply with apredetermined value to open the switch element when the output voltageis higher than the predetermined value.
 32. The control circuit asclaimed in claim 30, wherein the capacitive element is connectable to anoutput terminal of the switching power supply.
 33. The control circuitas claimed in claim 31, wherein the capacitive element is connectable toan output terminal of the switching power supply.
 34. The controlcircuit as claimed in claim 30, further comprising: a third resistor anda fourth resistor that are connected to voltage-divide an output voltageof the switching power supply; a comparator having a first inputterminal configured to receive the output voltage voltage-divided by thethird resistor and the fourth resistor, the comparator having a secondinput terminal configured to receive a reference voltage; and acontroller configured to control a switching transistor based on anoutput signal of the comparator, wherein the reference voltage input tothe second terminal of the comparator becomes a first voltage when anoutput of the comparator takes a first voltage level and a secondvoltage when the output of the comparator takes a second voltage level,and the third resistor is connectable between a positive terminal of theoutput terminal of the switching power supply and the first inputterminal of the comparator, and the control circuit has the capacitiveelement connectable in parallel with the third resistor.
 35. The controlcircuit as claimed in claim 31, further comprising: a third resistor anda fourth resistor that are connected to voltage-divide an output voltageof the switching power supply; a comparator having a first inputterminal configured to receive the output voltage voltage-divided by thethird resistor and the fourth resistor, the comparator having a secondinput terminal configured to receive a reference voltage; and acontroller configured to control a switching transistor based on anoutput signal of the comparator, wherein the reference voltage receivedby the second terminal of the comparator becomes a first voltage when anoutput of the comparator takes a first voltage level and to a secondvoltage when the output of the comparator takes a second voltage level,and the third resistor is connectable between a positive terminal of theoutput terminal of the switching power supply and the first inputterminal of the comparator, and the control circuit has the capacitiveelement connected in parallel with the third resistor.
 36. The controlcircuit as claimed in claim 30, further comprising: a third resistor anda fourth resistor that are configured to voltage-divide an outputvoltage of the switching power supply; an error amplifier having a firstinput terminal configured to receive the output voltage voltage-dividedby the third resistor and the fourth resistor, the error amplifierhaving a second input terminal configured to receive a referencevoltage; and a controller configured to control a switching transistorbased on an output voltage of the error amplifier, wherein thecapacitive element is insertable between an output terminal of the erroramplifier and the first input terminal of the error amplifier.
 37. Thecontrol circuit as claimed in claim 31, further comprising: a thirdresistor and a fourth resistor that are configured to voltage-divide anoutput voltage of the switching power supply; an error amplifier havinga first input terminal configured to receive the output voltagevoltage-divided by the third resistor and the fourth resistor, the erroramplifier having a second input terminal configured to receive areference voltage; and a controller configured to control a switchingtransistor based on an output voltage of the error amplifier, whereinthe capacitive element is insertable between an output terminal of theerror amplifier and the first input terminal of the error amplifier. 38.The control circuit as claimed in claim 30, wherein the output voltagedetection circuit comprises a fifth resistor and a sixth resistorconfigured to voltage-divide an output voltage of the switching powersupply; a second comparator; and a reference voltage source configuredto supply the second comparator with the reference voltage.
 39. Thecontrol circuit as claimed in claim 31, wherein the output voltagedetection circuit comprises a fifth resistor and a sixth resistorconfigured to voltage-divide an output voltage of the switching powersupply; a second comparator; and a reference voltage source configuredto supply the second comparator with the reference voltage.
 40. Thecontrol circuit as claimed in claim 38, wherein the output voltagedetection circuit comprises: a flip-flop circuit and a signal sourcethat is connectable to the flip-flop circuit.
 41. The control circuit asclaimed in claim 39, wherein the output voltage detection circuitcomprises: a flip-flop circuit and a signal source that is connectableto the flip-flop circuit.
 42. A switching power supply comprising: acontrol circuit of a switching power supply, the control circuitcomprising: a capacitive element; a switch element that is connectableto the capacitive element; and an output voltage detection circuitconfigured to compare an output voltage of the switching power supplywith a predetermined value, close the switch element to charge thecapacitive element when the output voltage is lower than thepredetermined value, and open the switch element when the output voltageis higher than the predetermined value.
 43. A switching power supplycomprising: a control circuit of a switching power supply, the controlcircuit comprising: a capacitive element; a switch element that isconnectable to the capacitive element; and an output voltage detectioncircuit configured to close the switch element to charge the capacitiveelement when the switching power supply is activated, and compare anoutput voltage of the switching power supply with a predetermined valueto open the switch element when the output voltage is higher than thepredetermined value.